TECHNOLOGY Computer program predicts fluid loss to thief zones

Aug. 14, 1995
I.M. Kutasov MultiSpectrum Technologies Inc. Santa Monica, Calif. A short computer program written in Fortran predicts the rate of fluid loss during drilling through a high-permeability formation (thief zone). Loss of circulation can be a serious problem in drilling, especially in deep wells. When fluid loss is detected, the drilling engineer has to decide if the rate of fluid loss can be tolerated without changes to the mud program or if the mud has to be treated with large quantities of
I.M. Kutasov
MultiSpectrum Technologies Inc.
Santa Monica, Calif.

A short computer program written in Fortran predicts the rate of fluid loss during drilling through a high-permeability formation (thief zone).

Loss of circulation can be a serious problem in drilling, especially in deep wells. When fluid loss is detected, the drilling engineer has to decide if the rate of fluid loss can be tolerated without changes to the mud program or if the mud has to be treated with large quantities of plugging materials.

In addition to predicting fluid loss, the program can also be used to predict pore fluid influx into a well if the pore pressure exceeds the mud pressure.

The program assumes that the loss of drilling fluids starts as the bit penetrates the top of an interval of high-permeability formations. The rate of fluid loss increases with time until the bit reaches the bottom of the thief zone and decreases thereafter.

The difference between the mud column pressure and the pore pressure is assumed to be the dominant factor causing the filtration at any depth. The impact of plugging materials (positive skin factor) is accounted for by the introduction of the effective well bore radius.1 The computer program determines the rate of fluid loss as a function of drilling fluid density, viscosity, compressibility, formation permeability, porosity, bit size, rate of penetration (ROP), thickness of the thief zone, and the current depth of the well bottom (bit position). The accompanying box lists the Fortran computer code.

Fig. 1(37448 bytes) is a fluid loss schematic diagram of a well drilled with a constant ROP. At time t = 0, the bit reaches the top of the thief zone at h = ho. As drilling continues, the mud filters into the thief zone, and a radius of the skin is created ( rs ). The rate of fluid loss will increase until the bit reaches the bottom of the thief zone ( h1 ) because the area exposed to filtration increases with depth. After the penetration of the thief zone, the rate of fluid loss will gradually decrease, because for any depth the rate of fluid loss per unit of length decreases with time and the exposed area of the thief zone is constant.

The mud weight is assumed to remain constant. The thief zone formation is assumed to be a uniform, homogeneous medium with high permeability. The porosity and permeability of the thief zone are typically unknown during actual drilling. Only the rate of fluid loss (q) can be recorded by the drilling operator.

This computer program generates a set of curves for values of q as a function of the current depth ( hb ) for various thief zone properties. Then, by the use of conventional methods of curve matching, the permeability and porosity of the thief zone can be estimated.2

Example problem

The following example demonstrates how this computer program can be used to predict the rate of fluid loss in a thief zone.

As the bit penetrated the 4,000-4,200 ft interval, the driller detected significant fluid losses. At 4,200 ft, the rate of fluid loss was about 75 gpm. The well was being drilled with an 11.0-ppg density mud at a pump rate of 300 gpm. Geological and geophysical investigations have shown that an interval of high permeability formations could be expected at a depth of 4,000-4,500 ft.

It was assumed that the formation porosity is 0.25 and the permeability between 300 md and 800 md. Other input data include the following: initial skin factor is 0 (no plugging), penetration rate is 50 ft/hr, compressibility is 0.0000036 1/psi, bit diameter is 1212 in., fluid viscosity is 27.0 cp, and the pore fluid density is 9.0 ppg. The casing shoe should be placed at a depth of 8,000 ft.

Tables 1(8559 bytes) and 2 show the input and output files for running the program with these data.

Table 2(36915 bytes) shows that the formation permeability is close to 650 md. As expected, the maximum value of the rate of fluid loss occurred as the bit penetrated the bottom of the thief zone.

The operator decided to treat the drilling mud with large quantities of plugging materials. The computer program was then used to estimate the efficiency of the plugging materials (Table 3)(8555 bytes). It is interesting to note that the increase in the skin factor drastically reduced the dependence of the fluid loss on formation permeability (Table 4)(33915 bytes).

References

1.Uraiet, A.A., and Raghavan, R., "Unsteady Flow to a Well Producing at Constant Pressure," Journal of Petroleum Technology, October 1980, pp. 1,803-1,812.

2.Kutasov, I.M., and Bizanti, M.S., "Fluid Losses While Drilling," Society of Petroleum Engineers paper No. 13963, 1984.

The Author

I.M. Kutasov is a senior research engineer with MultiSpectrum Technologies Inc. in Santa Monica Calif. He was a graduate faculty member in the petroleum engineering and geosciences department at Louisiana Tech University and worked for Shell Development Co. in Houston as a senior research physicist.

Kutasov's research interests include the temperature regime of deep wells, transient pressure flow analysis, and drilling in permafrost areas. He holds an MS in physics from the Yakutsk State University and a PhD in physics from O. Schmidt Earth Physics Institute in Moscow. Kutasov is a member of the Society of Petroleum Engineers.

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