Real-time geo-pressure analysis reduces drilling costs

William P. Kenda
Santa Fe Energy Resources
Houston

Steve Hobart
Knowledge Systems Inc.
Stafford, Tex.

Eamonn F. Doyle
Knowledge Systems Inc.
Bergen, Norway

• Equations [296,172 bytes]

The improved understanding of subsurface pressures achieved through real-time geo-pressure analysis enabled Santa Fe Energy Resources to save over $ 1.8 million in direct drilling costs.

Three practical case histories are examined in respect to the integration of measurement-while-drilling (MWD), seismic, and offset well data.

For the first case study, located in South Timbalier Block 179, real-time geo-pressure analysis monitored and verified the predrill evaluations, allowing Santa Fe to successfully deepen a well using a reduced mud density. This allowed the company to avoid mud-loss problems while eliminating an intermediate drilling liner.

For the second case study, located in Ship Shoal Block 365, the additional precision gained in the understanding of pore pressures and fracture gradients was instrumental in eliminating a casing string from the predrill casing plan.

The third case study, located in Mississippi Canyon Block 154, describes some limitations in regards to state-of-the-art of geo-pressure evaluation through petrophysical methods. The knowledge gained from the analysis of this well can be applied to improving the drilling efficiency of future wells in this area.

Real-time evaluation

When first introduced to the drilling industry, MWD technologies held great promise for real-time formation evaluation mainly because more timely formation evaluation facilitates better decision-making regarding drilling and completion practices.

The earliest MWD formation evaluation sensors used gamma ray and resistivity instruments. These were initially used for geologic correlation and rudimentary formation evaluation. However, early on, it was also recognized that the information from these sensors could also be used for pore-pressure and fracture-gradient estimation, facilitating mud-weight and casing-seat decisions, even during the drilling process.

Real-time pressure advantages

MWD data offer distinct advantages over offset well and seismic information used in predrill geo-pressure prediction. For instance, the distance from the offset wells, coupled with the geologic complexity, limits the confidence with which knowledge from the offset wells can be used towards evaluating pore pressures at a new drilling location.

In addition, although seismic information is generally more site-specific than offset well data, inherently it is of low resolution and sometimes of questionable value when applied to pore-pressure forecasting.

This is not intended to suggest that predrill studies using offset and seismic data are of no value in well planning, when in fact, they are of immense value. However, real-time geo-pressure analysis using MWD data is preferable because it is both site-specific and of high resolution. These differences in data quality, coupled with the timeliness inherent in a real-time analysis, can contribute to significant bottom-line savings in drilling costs.

Analysis methodology

For each of the wells in these case studies, Santa Fe initially determined mud weight and casing-program designs based solely on correlation to offset well information. Subsequent to the initial designs, seismic interval velocity data were analyzed for preliminary pore-pressure and fracture-gradient profiles.

Recognizing the potential for avoiding unnecessary drilling costs, Santa Fe decided to utilize a real-time analysis of MWD data throughout the drilling process to continuously review and refine mud weights and casing plans.

Preliminary analyses

Seismic interval velocity data were analyzed using the familiar Exponent method for acoustic log data (Equation Box, Equation 1). ¹ The normal pore pressure gradient used for the analysis was equivalent to a 9 ppg mud density.

Due to the absence of local density logs, the overburden gradient (OBG) required for these analyses was obtained through the integration of synthetic density data.

Synthetic density data were obtained either from the Gardner equation (Equation 2),² which is used to transform velocity to density, or from an empirical "Average Gulf Coast Formation Density" equation, published by Martin Traugott of Amoco Corp. (Equation 3).³

The Gardner equation, in general, seems to slightly underestimate density. Therefore, it was used only in the Mississippi Canyon 154 case study where the less dense sediments are younger than average Gulf Coast formations. The recently published Amoco formula was applied to the remaining two wells.

Real-time analysis

The real-time pore-pressure analysis was performed at the well sites using a commercially available system4 linked to the MWD contractor's computer, using an RS-232 cable and WITS (wellsite information transfer standard) Level 0 protocol.

The well-site analyst used the system to identify shale depths with the MWD gamma ray (GR) in order to establish shale resistivity trends. The analyst then applied the Exponent method to estimate pore pressure from the resistivity (Equation 4) using this pore pressure to estimate the fracture gradient (Equation 5).^{1 5}

The Exponent method, like most other methods, is a form of Terzaghi's Law, which states that the overburden stress is balanced by the pore-fluid stress and the grain-to-grain stress in the rocks. This grain-to-grain stress is often referred to as the "effective stress" or the "matrix stress." When the overburden and effective stresses can be estimated, the pore fluid stress can be estimated using this relationship. By dividing each of the stress terms by the vertical depth, a stress gradient is obtained, which is dimensionally equivalent to a density. From here, the mud density required to balance the pore-fluid pressure can be obtained.

For the real-time analysis using the MWD resistivity data, the overburden was estimated using the same empirical methods as applied to the prespud seismic analysis (Equations 2 and 3).

The effective stress was estimated in real-time using the method popularized by Ben Eaton (Equation 4).1 In this "Exponent method," the effective stress is related to the normal effective stress by a factor which is the ratio of the actual shale resistivity to the normal shale resistivity. An empirical exponent of 1.2 is then applied to this ratio. This relationship is then solved for the pore pressure.

The term "normal" in this context refers to the condition of "normal pore pressure," which refers to the pressure exerted by an unconfined column of pore fluid. For the U.S. Gulf Coast, this is assumed to be equivalent to a 9 ppg mud density.

The resistivities of normally pressured shales form a trend that is extrapolated to overpressured zones in order to provide the necessary normal shale resistivity input for the analysis.

In order to estimate the fracture stress gradient in real-time, use was made of the general assumption that the ratio of the vertical effective stress to the minimum horizontal effective stress can be characterized by Equation 5, called the matrix stress ratio.

The horizontal effective stress is equal to the minimum horizontal stress (considered equal to the fracture stress) pressing the rocks together, less the pore fluid pressure. The form of the fracture gradient formula that was preferred for these Gulf Coast wells states that the matrix stress ratio is equal to an empirically derived Poisson Ratio divided by one minus the Poisson Ratio (Equation 6).⁵ The exact value of this Poisson Ratio is dependent on the depth below the mud line (Equation 7).

The pore pressure and fracture gradient were estimated in real-time and continuously compared to the mud density being used during the drilling process, as well as to the predrill analysis results.

By incorporating this analysis with the well behavior at the time, decisions could be efficiently made regarding the casing setting depth and whether the mud density required adjustment. The predrill seismic analysis, using the pore pressure magnitudes recalibrated from the real-time data, provided a low-resolution indicator of the pore pressures to be expected ahead of the bit.

Well deepening

For the first case history, the South Timbalier 179 well, the geologic objective was located at a depth of 17,000 ft (5,200 m) ( Fig. 1 [182,717 bytes]). A prespud analysis of the offset data, performed by the turnkey drilling contractor, provided the basis for the mud-weight and casing-configuration programs to be used in reaching this objective. The well was drilled to 16,400 ft, at which point the well was temporarily abandoned because of mud losses.

The well had been cased to 16,000 ft with a leak-off test at the shoe of 18.6 ppg equivalent mud weight. At 16,400 ft, with 18.4 ppg mud in the hole, mud losses were experienced. Mud losses were still evident at a mud weight of 18.1 ppg. Based on the prespud analysis, Santa Fe decided that further drilling was too risky and the well was temporarily abandoned.

The drilling program was subsequently reviewed, taking into account data obtained from wire-line resistivity and sonic logs down to 16,000 ft. Using a commercially available geo-pressure analysis software package,4 the pore pressure at 16,000 ft was estimated to be only 16.8 ppg.

Using seismic-derived interval transit-time data, the pore pressure at 17,000 ft was estimated to be 17.2 ppg. The new analysis indicated that the well could be drilled to TD with a mud weight in the range of 17.3-17.7 ppg.

The hole was reopened to 16,400 ft while logging with MWD resistivity and GR tools. The MWD logging data were transmitted in real-time to the surface.

The real-time data were merged with the wire-line data for a continuous real-time, pore-pressure analysis (Fig. 2 [118,506 bytes]). This analysis confirmed the projection from seismic data and provided confidence that the objective at 17,000 ft could be reached.

The real-time analysis of MWD resistivity data was maintained to TD in order to assist in optimizing the mud weight. The TD was reached using a 61/2-in. drill bit and a 17.8 ppg mud weight, removing the need for the planned 51/2-in. drilling liner and providing cost savings associated with casing, cementing, and rig time.

In addition, by maintaining a larger hole size, drilling difficulties normally encountered with a slimmer hole were avoided (Fig. 3 [116,893 bytes]). This also facilitated the use of larger tubulars for the gravel-pack completion, ensuring the maximum possible production rates. By maintaining the larger hole size, additional rig time was saved because it was not necessary to pick up a smaller size drill pipe to finish the well. Santa Fe estimated total cost savings of $634,750.

Liner elimination

The prognosed TD for the second case history, the Ship Shoal 365 well, was 14,000 ft. Based on the mud-weight and casing programs from offset wells, it was considered likely that a 75/8-in. drilling liner would have to be run to TD.

However, the seismic-data analysis indicated the possibility that pore pressures would not be as high as suggested by the offset well data. To resolve this issue, Santa Fe utilized real-time pore-pressure analysis.

MWD GR and resistivity data were collected and analyzed immediately below the 30-in. casing shoe (Fig. 4 [133,005 bytes]). This provided sufficient data to establish the resistivity trend of normally compacting shales used in real-time pore-pressure analysis.

Drilling parameters and the real-time pore pressure analysis of the MWD resistivity data confirmed that the predrill analysis for the seismic data was correct-that the pore pressures were not as high as expected based on the offset well data.

This removed the need for a liner and allowed the operator to deepen all casing points. This led to subsequent savings in casing hardware and rig costs while allowing for optimum hole sizes for completion and production needs (Fig. 5 [135,479 bytes]). The related cost savings, calculated by Santa Fe, totaled $1,177,750.

Improved analysis

Following the above two wells, another well, the Mississippi Canyon 154, was drilled in young sediments off the mouth of the Mississippi River. A prespud analysis of the seismic data was conducted and compared to a prognosis using offset well data ( Fig. 6 [110,119 bytes]).

Based on the analysis, it appeared that an intermediate drilling liner again could be saved, as with the Ship Shoal 365 well. MWD GR and resistivity data were collected and analyzed in real-time, but did not confirm the seismic data analysis.

However, the analysis indicated a lower pore pressure than that which was compared to the offset wells. Based on this evaluation, the setting of the 95/8-in. casing was deferred from the original planned depth of 8,200 ft.

At 8,940 ft, however, a kick was taken indicating a pore-pressure gradient of 13.3 ppg with a mud weight of 12.5 ppg. The pore pressure calculated from MWD resistivity was only 11.7 ppg, and the pore pressure from the seismic was 10 ppg (Fig. 7 [112,094 bytes]).

The mud weight was raised to 13.8 ppg and the 95/8-in. casing was subsequently set at 8,800 ft. Because of the kick, the original pore pressure analysis, based on offset well data, was given more credence than the real-time data analysis. The mud weight used from that point onward was in accordance with the original prognosis.

Trend-line recalculation

Based on the magnitude of the kick, a new normal-compaction, trend-line position was calculated. The position of the new trend-line was so radically different from the original position that the original calculation of the OBG was in question.

However, these adjustments to the analysis did not improve the capability to accurately predict the relationship between well and formation pressures, i.e., the degree of over-balance or under-balance. The subsequent drilling was characterized by mud losses. In addition, there was no connection gas even when the pore pressures derived from the new trend-line and MWD resistivity data approached the mud weight.

The conclusion drawn from the weight of these data was that the excess pore pressure manifested in the kick was not the result of simple undercompaction. More likely, the sand in which the kick was taken had been charged with pressures from a deeper formation, possibly transmitted by a fluid-conductive fault.

If it were to be presupposed that this rapid pressure increase was in fact caused by undercompaction, its detection would have been compromised by the silty nature of the overlying formation. The reason for this is that reliable petrophysical pressure evaluation relies on the input of data from clean shale lithologies (Fig. 8 [85,221 bytes]).

There is also the possibility that a change in formation fluid salinity above the sand zone could have affected the resistivity-based compaction model. A logging-while-drilling sonic measurement would not have been affected by this phenomenon.

Lessons learned

Based on experience gained from these wells, the following insights became evident:

1. Site-specific, real-time pore-pressure analysis, provided by linking MWD formation evaluation services with analytical software, can be used for optimizing mud weights in addition to fine tuning drilling hydraulics and casing programs, sometimes resulting in spectacular bottom-line benefits.
2. There are many possible mechanisms for abnormally high pore pressures, several of which are not detectable using either seismic data or MWD logging measurements. Fortunately, the most prevalent cause, undercompaction, is amenable to analysis using this data.
3. Readjustment of the normal-compaction trend line on the basis of a kick can be erroneous if the overpressure in the formation is caused by some factor other than undercompaction. An understanding of the geologic structures and subterranean plumbing is necessary for a complete picture.
4. The incremental cost of real-time pore pressure analysis, relative to the potential positive impact on the well economics, is generally a worthwhile investment.

Acknowledgment

The authors wish to thank Santa Fe Energy Resources and Knowledge Systems Inc. for providing the time and resources required to publish this article.

References

1. Eaton, B.A., "Graphical Method Predicts Geopressures Worldwide," World Oil, June 1972, pp. 51-56.
2. Gardner, G.H.F., Gardner, L.W., Gregory, A.R., "Formation Velocity and Density-The Diagnostic Basis for Stratigraphic Traps," Geophysics, 39, No. 6, June 1974, pp. 2085-95.
3. Traugott, M., "Pore/fracture pressure determinations in deep water," World Oil, Deepwater Technology Special Supplement, August 1997, pp. 68-70.
4. Greenberg, J., "Managing Loss-of-control in Deepwater Drilling," Offshore Magazine, April 1998, pp. 58-60.
5. Eaton, B.A., and Eaton, T.L., "Fracture Gradient Prediction for the New Generation," World Oil, October 1997, pp. 93-100.

The Authors