Sonic-while-drilling tool detects overpressured formations

Kai Hsu, Gerald N. Minerbo
Schlumberger Anadrill
Houston

Mohamed Hashem
Shell Offshore
New Orleans

Clarke L.Bean
Chevron USA Production
New Orleans

Richard Plumb
Schlumberger Cambridge Research
Cambridge, U.K.

Real-time detection and estimation of overpressured formations is now possible using sonic and other logging-while-drilling (LWD) measurements.

Data acquired from an overpressured formation in the Gulf of Mexico (GOM) are used to demonstrate this capability.

The detection and evaluation of overpressured formations are critical for the exploration and production of hydrocarbon reservoirs.^{1 2}

Formation pressures can induce water influx from adjacent overpressured shales and provide an additional driving mechanism for hydrocarbon production.²

In active sedimentary basins, overpressures are generated by undercompaction or fluid expansion. Overpressure tends to develop where fluid flow is severely restricted for long periods of geologic time. In the U.S. Gulf Coast area, thick Tertiary shales form permeability barriers that trap fluids within the shale and underlying sediments.

Overpressure and undercompaction develop together when pore fluids cannot escape in response to increased sediment loading. Pore pressures increase as the trapped fluid supports the increased sediment load. The increased pore pressure reduces the effective overburden stress leading to a state of undercompaction.

In similar basinal settings, overpressures can be generated by fluid expansion. Geologic processes that give rise to fluid expansion include aqua-thermal heating, smectite transformation to illite, and kerogen maturation.³ In many cases, detection and evaluation of overpressure requires analysis of regional and local geological information combined with borehole measurements.

Knowledge of expected overpressured formations provides the ability to efficiently and safely drill wells with correct mud weights,⁴ engineer casing programs, and perform completions.

There are many techniques available for detecting and estimating overpressured formations.^{1 3 5-10}

Techniques include interpretation of drilling parameters and wire line logging data. Although wire line analysis and evaluation is considered an after-the-fact, post-drilling process, the availability of sonic and other LWD measurements provides a unique opportunity to perform real-time, wire line-type analysis at the well site.

Case study

The study case is an exploratory well located 18 miles offshore Louisiana. It was designed to test a middle Miocene section deeper than 15,000 ft. Offset well data provided evidence of an overpressured, lower Pliocene to upper Miocene sandstone and shale sequence from 8,500 to 11,000 ft. This created an ideal situation for using LWD sonic and other LWD tools to predict overpressured regimes.

Shales with a few clean or thinly bedded sandstones comprise the predominant lithology.

(Table 1) [32,465 bytes] shows the drilling summary for the GOM well. At 4,802 ft, 13 3/8-in. casing was set and drilled out using a 12 1/4-in. bit. The LWD sonic tool was placed in the drillstring at 8,783 ft. There were three bit runs over a 9 day period.

For the entire logged interval, the hole deviated less than 2°. A water-based polymer mud with lost-circulation material and barite additives was used for drilling. Mud weights ranged from 11 ppg for Bit Run 1 to 13 ppg for Bit Run 3.

The pendulum-type bottom hole assembly (BHA) consisted of a compensated dual-resistivity (CDR) tool, measurement-while-drilling (MWD) telemetry system, and Schlumberger Anadrill's Isonic Ideal LWD sonic tool.

The LWD sonic tool, placed above the MWD unit and below a stabilizer, was positioned approximately 68 ft from the bit. A suite of wire line logging tools including lithodensity, compensated-neutron, and long-spaced sonic were run 2 weeks after the third bit run.

Data acquisition

An LWD sonic compressional tool was first reported in 1994.11 Since then, many successful real-time and memory Dt logs have been recorded in hard and soft-rock formations. ¹²

The LWD sonic tool consists of one transmitter and an array of four receivers. The distance between the transmitter and receivers is similar to that of wire line array tools.

The tool electronics store sonic waveforms and operate under the control of a downhole microprocessor. The transmitter is fired, and the four waveforms are recorded. The waveform amplitudes are then digitized with an analog-to-digital conversion using a 12-bit dynamic range. Then, the digitized data are added to a waveform stack which increases the signal-to-noise ratio.

Experience indicates that drilling-induced noise is large for hard formations like limestone. Therefore, waveform stacking becomes indispensable, because the stacking operation can significantly reduce the drilling-induced noise.¹²

Next, the stacked waveforms are filtered within a finite-frequency band, where drilling noise is low, and collar arrivals are greatly reduced by the tool's attenuation section. Finally, the filtered waveforms are saved into memory.

In addition to waveform recording, the tool performs the downhole waveform processing necessary for extracting the compressional Dt during the drilling process. A system consisting of a microprocessor and a digital-signal processor (DSP) performs waveform filtering, slowness-time coherence (STC) processing,¹³ and compressional Dt selection (labeling) obtained from the STC output. STC is a correlation technique which detects coherent signals in the waveforms and computes a measure of similarity or coherence for each one of them. The measureable Dt range is 40-180 µ sec/ft. Once the compressional Dt is extracted, it is transmitted real-time to the surface through the mud-telemetry system, thus generating a sonic log while drilling. Prior to drilling, the data rate is set up so that the sample spacing of the sonic log, or the distance between two consecutive Dt values, ranges from 6 in. to 1 ft based on the anticipated rate of penetration (ROP).

Overpressure detection

Costly misinterpretations can be avoided by studying a combination of overpressure indicators acquired during drilling and from wire line logging. Traditionally, the sonic log is used to provide a means for estimating formation porosity. The measured Dt is primarily a function of porosity and lithology. For example, in relation to shale, the Dt log responds to porosity variations. Normally, the Dt decreases with increasing burial depth because of the overburden stress, meaning that porosity decreases as a function of depth. The values of Dt that follow this trend-line indicate that formation pressures maintain normal-compaction pressures. If overpressured formations are encountered, the Dt data points will diverge from the normal trend toward abnormally high values. The amount of divergence from the normal trend can be used to estimate the abnormal formation pressure.^{3 5-9} The availability of compressional Dt and other LWD measurements makes real-time detection and estimation of overpressured formation possible at the well site. Fig . 1 [ 29,696 bytes] shows the LWD sonic Dt log obtained during tripping in and drilling. The wire line long-spaced sonic (LSS) log is also shown for comparison. Besides the combined LWD interval from 8,700 to 10,600 ft, the LWD sonic tool also recorded data for each bit trip. The Dt values computed while tripping in the hole are shown for three bit runs. Despite coarse Dt sampling values and marginal depth controls caused by tripping in at rapid speeds (2,500-3,600 ft/hr), a well-defined trend of decreasing Dt values was established, and corresponded to increasing depth (Fig. 1). This trend was similar to the wire line sonic log of track.³ The dashed line approximating the shale Dt wire line values indicates a normal compaction trend from 5,000 to 8,500 ft. At about 9,580 ft, the LWD sonic Dt began to diverge from the normal compaction trend, indicating the possibility of upcoming overpressured formations. The deeper the well, the larger the Dt divergence became. The wire line sonic log also showed a similar divergent trend.

Drilling parameters and other LWD measurements were used to verify the detection of overpressured formations. (Fig. 2) [34,404 bytes] shows a standard presentation of the gamma ray (GR) in Track 1, phase-shift resistivity and attenuation resistivity in Track 2, and the LWD sonic log in Track 3.

The GR log indicates that the entire interval below 9,375 ft is primarily shale. It has been proposed that under normal compaction, the shale resistivity increases with depth, because porosity decreases. However, in an overpressured shale, the resistivity shows a departure from the normal trend toward lower-than-normal values. This is because of the increasing porosity and salt water content in the shale.

The two resistivity logs in Fig. 2 show this decreasing resistivity trend below 9,580 ft, where the LWD sonic Dt values also depart from the normal trend.

ROP and mud-weight

The ROP is useful for detecting overpressured formations also. However, ROP is affected by changes in lithology, bottom hole cleaning, mud properties, bit type and condition, and weight-on-bit. Assuming that all other factors remain constant, ROP generally increases with increasing pore pressure.

In Fig. 2 it is seen that the ROP began increasing below 9,580 ft. However, given the same lithology of shale, this provided additional evidence of overpressure buildup.

In addition, a steady increase of mud weight from 11 ppg to 13 ppg was recorded in the drilling report, indicating the crews needed to increase mud density Table 1.

The report also described hole caving at about 10,390 ft beginning with Bit Run 3, indicating an underbalanced situation. However, an attempt to increase the mud weight from 11.5 to 12.1 ppg was not successful in balancing the hydrostatic and formation pressure and the hole continued to cave in.

Eventually, the mud weight was raised to 12.5-13.0 ppg in order to maintain borehole integrity, and it was noted that gas-cut mud was present.

Wire line logging

Besides the LSS sonic log shown in Fig. 1, wire line litho-density and compensated-neutron logs were also run. (Fig. 3) [34,763 bytes] shows wire line neutron-porosity and bulk-density logs in Track 2. This log also shows sonic porosity derived from the Wyllie equation. ¹⁴ Of particular interest is the marked transition at 9,600 ft for which the neutron-porosity increases from low to high values.

The GR and caliper logs are shown on Track 1. The caliper log indicates that the hole was badly washed out in many places at the time of wire line logging. As mentioned earlier, drilling stopped at 10,390 ft, and extensive circulation was required to increase the mud weight. As a result, the hole was badly deteriorated from 10,300 to 10,400 ft.

After the mud weight was increased to correct the underbalanced condition, hole conditions improved. The enlarged hole implied that there was sloughing shale.

There are two primary reasons for sloughing shale in U.S. coastal areas:

1. Overpressured shale conditions
2. Shale hydration or swelling.⁴

With the evidence shown, it is most likely that the overpressured shale caused the formation to slough, resulting in the creation of a badly washed-out hole. Dt and quality control logs(Fig. 4) [36,525]shows a standard presentation of an LWD sonic log in Track 2, including quality-control, stacking indicators in Track 1. The image shown on Track 3 is the STC projection with coherence levels color coded from light green (coherence = 0) to scarlet (coherence = 1). The solid curve overlying the display is the Dt log. The projection is a quality control display for the Dt log because the log passes through a continuous, high-coherence region on the display. It also indicates the data quality and how coherent the formation arrival is. With higher coherence values, data quality improves, resulting in more reliable Dt values. Other coherent arrivals, such as fluid modes shown in red 200 ? sec/ft), are easily identified using this display.

Overall, the projection on Track 3 shows that the formation arrival's coherence is high, except for the initial 70 ft of each bit run. Because the distance between the sonic measurement and the bit was approximately 68 ft, the sonic tool was actually recording data from the 70-ft interval drilled in the prior bit run.

Because of prolonged mud exposure and circulation time, and the possibility of an underbalanced condition, the hole at the beginning of each bit run was enlarged. Further confirmation is supported by computer simulations showing that enlarged hole diameters support a stronger fluid arrival. The stronger, coherent-fluid arrival is clearly seen on Track 3, with the slowness approaching 200 msec/ft at the beginning of each bit run.

The stacking indicator and the coherence value of the Dt log are shown in Track 1 of Fig. 4. The LWD sonic tool was programmed to acquire waveforms with and without stacking, and were evenly spaced over the entire interval. The stack size alternated between 1 and 32 (red bar) as shown on the first track of Fig. 4.

The coherence value ranged from 0.7 to 0.95 and correlated well with the stacking indicator. The higher coherence values conformed with the stacked waveforms. There was one exception at 10,320-10,400 ft, which resulted in a degraded formation signal. This was probably caused by deteriorated hole conditions.

LWD and wire line sonic comparison

(Fig. 5) [46,887 bytes] compares the wire line variable-density logs (VDL) and the LWD VDL recorded during drilling. The raw wire line waveforms have a wider frequency bandwi Dt h and were filtered to the same frequency band as the LWD sonic waveforms.

The formation compressional signals varied with depth and showed similar characteristics for both VDLs. Generally, they were in excellent agreement. The weak signals in early portions of the LWD waveforms are from electrical cross-talk.

Residual noise from drilling is visible before compressional arrival as shown in the intervals 8,750-8,950 ft and 9,150-9,350 ft. The waveforms in these two intervals were acquired without stacking.

Of particular interest are the initial 70-ft intervals for Runs 2 and 3. In these zones, the weaker formation compressional arrival and the stronger fluid arrival were observed because of the enlarged hole conditions resulting from previous drilling.

Also shown in Fig. 5 is a comparison of LWD and wire line sonic logs. There is a consistent difference between the two in the overpressured formation below 9,580 ft. The trend shows that the LWD sonic measurements are a few m sec/ft slower than the wire line sonic. Furthermore, this difference increased with depth.

In contrast, the two measurements are close in value and nearly overlie each other in several intervals above 9,400 ft, shown at an expanded scale in (Fig. 6) [27,920 bytes]. The two Dt measurements agree well in the water-sand interval from 9,240 to 9,330 ft, and from 9,030 to 9,090 ft. The comparison indicates that the differences between the two measurements were probably not caused by differing tool response. Sonic measurements are known to be affected by washouts and cave-ins. Washouts and cave-ins cause the measured Dt values to oscillate around the true-formation Dt values.

Although the LWD sonic log in Fig. 5 was not borehole compensated, and the wire line LSS sonic was, the consistent difference between the two measurements observed in the overpressured formation was probably not caused by washouts or cave-ins for the following reasons.

First, the hole should have been in better condition while drilling, therefore the requirement for borehole compensation was minimal. Second, the consistently slower trend of the LWD measurements conflicted with the effect of washouts on the Dt values, which should have made the uncompensated Dt values fluctuate slower or faster around the compensated wire line measurements.

Because the slower trend of the LWD sonic was persistent (9,600-9,800 ft), it was concluded that borehole compensation was probably not the cause for the discrepancy. Supporting evidence is illustrated by the close agreement between the two logs above the overpressured formation (Fig. 6).

Two different data processing methods were used to obtain the LWD and wire line sonic logs in Fig. 5. Although the STC processing was used for LWD, the wire line log was computed using a first-motion-detection scheme (FMD). This is triggered by waveform amplitudes exceeding a selected threshold.

There were measurement-discrepancy concerns which can be caused by different processing methods. However, the wire line LSS waveforms were filtered to the same frequency band as the LWD waveforms, and processed with STC.

(Fig. 7) [22,024] shows the comparison of the wire line FMD and STC logs in Track 1 for the interval 9,600-10,000 ft. Wire line sonic data for both the FMD and STC logs are similar. As an additional processing approach, peaks (or troughs) were manually picked along the sonic-array compressional moveout for each depth.

The Dt values were calculated from the slope of the line using a least-squares fit through the selected peaks. The advantage of the arrival-picking method lies in the user's ability to visually examine both the quality of the data and the validity of a linear moveout across the receiver array. The Dt log generated by the picking method is shown on the right track of Fig. 7 with the STC-processed log. Although somewhat noisier, the manually picked log agrees well with the STC-processed log. Thus, it was deduced that the discrepancy between the LWD and wire line measurements was not caused by differing methods in processing, and it was concluded that the difference in Dt between wire line and LWD measurements was most likely caused by changes in formation properties over time.Interpretation of Dt Fig. 5 shows a comparison between the compressional slowness measured by the wire line and LWD sonic tools. The measurements agree in the depth interval from 8,800 ft to the top of the overpressured section at about 9,580 ft (Fig. 1). From 9,580 to 10,200 ft, both logs show a steady increase in Dt values that is consistent with increasing overpressure. However, the wire line tool, run 2 weeks after drilling, is systematically lower then the LWD tool. The time-dependent decrease of Dt implies that the compressional modulus in the overpressured shale section has increased within a radius of at least 12 in. of the well bore.¹⁵ This change could result from processes associated with drilling the well, including plastic shale deformation from compaction, increase of vertical effective stress,¹⁶ or both.

Because of the low permeability, the deformation of overpressured shale is expected to be time dependent.¹⁷ Drilling a well induces shale stress concentrations within a few inches from the well bore. If the drilling fluid's pressure is equal to or greater than pore pressure, the presence of the well will cause an increase in the effective mean-stress.

The long-term response of shale to higher mean-stress produces a decrease in porosity and a stiffening of the bulk-frame modulus. These effects tend to decrease Dt . This plastic deformation is restricted to the near well bore region.

The vertical effective stress could increase if pore pressure near the well bore decreased relative to the virgin in situ pressure. This situation could occur if the drilling fluid pressure was less than the pore pressure. Initially, there would be a strong pore-pressure gradient near the well bore.

Over time, the vertical effective stress would increase as the near-well bore pore pressure gradient decreased. This mechanism is appealing because it can affect the shale's Dt a foot or more away from the well bore. The affected depth at the time of logging will depend on the formation permeability, magnitude of the pressure difference, length of time since the well was drilled, and rock property considerations.

The caliper log in Fig. 3 shows that the well bore was enlarged to 15-20 in. by the time wire line logging took place. Hole enlargement in the overpressured section was probably created by shale fragments being blown off the borehole wall because of excess formation pressure.

This was recognized in the well cuttings by curved, splinter-shaped shale pieces found in the drilling returns throughout this portion of the well.

Time-dependent drilling fluid invasion into the shale and shale swelling mechanisms were initially predicted to increase Dt . However, Dt decreased. The overpressured shales penetrated by this well were reported to have a high-volume fraction of smectite clay minerals and a pore-fluid salinity four or five times greater than the drilling mud. This shale would be expected to swell with increasing porosity when in contact with a relatively fresh drilling mud, leading to an increase in Dt and is confirmed by laboratory experiments.¹⁸ In addition, theory and experiments show that substituting a saline pore fluid with a less saline drilling fluid (invasion) would also tend to increase Dt .^{19 20} Unfortunately, the effect of shale swelling and fluid substitution did not conform to what was observed. Given limited physical information about the shale, it was concluded that the time-dependent decrease of Dt could be caused by an increase of the vertical effective stress and a concomitant reduction of porosity near the well bore. Although swelling shale and invasion of drilling fluids are mechanisms that may have been at work, they cannot be the dominant mechanisms because both tend to increase, rather than decrease, Dt .

Acknowledgment

Special thanks to Shell Offshore Inc. and Chevron USA Production Co. for providing field test opportunities, assistance, and the release of data. We would also like to thank Gopa De of Chevron Petroleum Technology Co. for her inputs and comments.

References

1. Fertl, W.H., Abnormal Formation Pressures: Implications to Exploration, Drilling and Production of Oil and Gas Resources, Elsevier Scientific Publishing Co., New York, 1976.

2. Fertl, W.H., and Chilingarian, G.V., "Importance of Abnormal Formation Pressures," Journal of Petroleum Technology, pp. 347-54, April 1977.

3. Bowers, G.L., "Pore Pressure Estimation from Velocity Data: Accounting for Overpressure Mechanisms Besides Undercompation," SPE Drilling and Completion, pp. 89-95, June 1995.

4. Moore, P.L., Drilling Practices Manual, Petroleum Publishing Co., Tulsa, 1974.

5. Hottman, C.E., and Johnson, R.K., "Estimation of Formation Pressures from Log-derived Shale Properties," Journal of Petroleum Technology, June 1965.

6. Matthews, W.R., and Kelley, J., "How to predict formation pressure and fracture gradient from electric and sonic logs," OGJ, pp. 92-106, Jan. 20, 1967.

7. Eaton, B., "The Effect of Overburden Stress on Geopressure Prediction from Well logs," Journal of Petroleum Technology, pp. 929-34, August 1972.

8. Eaton, B., "The Equation for Geopressure Prediction from Well Logs," SPE paper 5544 presented at the Society of Petroleum Engineers 50th Annual Meeting, Dallas, Sept. 28-Oct. 1, 1975.

9. Eaton, B., "Graphical Method Predicts Geopressure Worldwide," World Oil, pp. 100-104, July 1976.

10. Bellotti, P., and Gerard, R.E., "Instantaneous Log Indicates Porosity and Pore Pressure," World Oil, pp. 90-94, October 1976.

11. Aron, J., Chang, S.K., Dworak, R., Hsu, K., Lau, T., Masson, J-P., Mayes, J., McDaniel, G., Randall, C., Kostek, S., and Plona, T., "Sonic Compressional Measurements While Drilling," SPWLA 35th Annual Logging Symposium, Tulsa, June 19-23, 1994.

12. Aron, J., Chang, S.K., Codazzi, D., Dworak, R., Hsu, K., Lau, T., Minerbo, G., and Yogeswaren, E., "Real Time Sonic While Drilling in Hard and Soft Rocks," SPWLA 38th Annual Logging Symposium, Houston, June 15-18, 1997.

13. Kimball, C.V., and Marzetta, T.L., "Semblance Processing of Borehole Acoustic Array Data: Geophysics," Vol. 49, No. 3, pp. 274-81, March 1984.

14. Wyllie, M.R.J., Gregory, A.R., Gardner, L.W., and Gardner, G.H.F., "Elastic wave velocities in heterogeneous and porous media," Geophysics, Vol. 21, No. 1, p. 41, January 1956.

15. Dvorkin, J., and Nur, A., "Elasticity of High Porosity Sandstones: Theory for Two North Sea Data Sets," Geophysics, Vol. 61, No. 5, pp. 1363-70, September 1996.

16. Tosaya, C.A., "Acoustic Anisotropy of Cotton Valley Shale," SPE paper 10995 presented at the Society of Petroleum Engineers 57th Annual Fall Technology Conference and Exhibition, New Orleans, p. 13, Sept. 26-29, 1982.

17. Detournay, E., and Cheng, A.H-D, "Poroelastic Response of a Borehole in a Non-hydrostatic Stress Field," Int. Journal of Rock Mech., Min. Sci. and Geomech., pp. 171-82, January 1988.

18. Reid, P., Personal Communications, Schlumberger Cambridge Research, 1997.

19. Murphy, W.L., Schwartz, L.M., and Hornby, B., "Interpretation of Vp and Vs in Sedimentary Rocks," SPWLA 32nd Annual Logging Symposium, Midland, June 16-19, 1991.

20. Liu, X. Vernik, L. and Nur, A., "Effects of Saturating Fluids on Seismic Velocities in Shale," 64th Society of Exploration Geophysicists Annual Meeting, Oct. 23-27, 1994.

The Authors