PROGRAM PREDICTS RESERVOIR TEMPERATURE AND GEOTHERMAL GRADIENT

I.M. Kutasov
Consultant
Los Angeles

A Fortran computer program has been developed to determine static formation temperature (SFT) and geothermal gradient (GG). A minimum of input data (only two shut-in temperature logs) is required to obtain the values of SFT and GG (Table 1).

Modeling of primary oil production and designing enhanced oil recovery (EOR) projects requires knowing the undisturbed (static) reservoir temperature. Furthermore, the bottom hole circulating temperature (BHCT) is an important factor affecting a cement's thickening time, Theological properties, compressive strength development, and set time.

To estimate the values of BHCT, the geothermal gradient should be determined with accuracy."

Recently we obtained an approximate analytical solution which describes the shut-in temperature behavior. 3 4

To determine the geothermal gradient, the computer program (see box) calculates the values of the SFT (Tf1, Tf2) at two different depths (H1, H2)- Thus the geothermal gradient is:

GG = (Tf2 - Tf1)/(H2 - H1)

FIELD EXAMPLE

Data for the Imperial ADGO Well P-25 were published by the Geothermal Service of Canada.5 The well was spudded on Jan. 2, 1973, and shut in on Mar. 28, 1975. After a 36 day shut in, downhole temperatures recorded on May 3, 1975, were as follows:

336 ft, 46.09 F.
530.8 ft, 46.90 F.
396.1 ft, 47.19 F.
791.0 ft, 47.70 F.
985.9 ft, 49.21 F.

After a 125 day shut in, on July 30, 1975, the temperatures were measured again and found to have decreased as follows:

336.0 ft, 42.49 F.
530.8 ft, 43.20 F.
596.1 ft, 43.70 F.
791.0 ft, 44.80 F.
985.9 ft, 46.20 F.

The well has a 17.5-in. diameter and was drilled to a total depth of 8,327 ft.

The input and output files are presented in Tables 1 and 2. The value of 0.04 sq ft/hr was found to be a good estimate for thermal diffusivity of sedimentary rocks. 6

To obtain the accurate values of GG, the depths H1 and H2 should be selected within a lithologically homogeneous well interval. In our case, the lithological profile of the well is not known. Therefore we consider this example only as an illustration.

NOMENCLATURE

The nomenclature used in the program is as follows:

NWELL = Well no.

DWi = Well diameter, in.

HTOTf = Total vertical depth, ft

TITOTd = Total drilling time, days

TIS1d = Shut-in time for the first temperature log, days

TIS2d = Shut-in time for the second temperature log, days

H1f = Vertical depth 1, ft

H2f = Vertical depth 2, ft

TEM11 =Temperature at H1f from the first log, F.

TEM12 = Temperature at H1f from the second log, F.

TEM21 = Temperature at H2f from the first log, F.

TEM22 = Temperature at H2f from the second log, F.

DIF = Thermal diffusivity of formations, sq ft/hr

TEFOR1 = Temperature of formations at H1f, F.

TEFOR2 = Temperature of formations at H2f, F.

GGX = Geothermal gradient/100 ft, F.

GX = Geothermal gradient calculated from the second temperature log, F,

GRAT = Ratio of GX to GGX

REFERENCES

Kutasov, I.M., and Targhi, A.K., "Better BHCT estimations possible," OGJ, May 25, 1987, pp. 71-73.
Beirute, R.M., "A Circulating and Shut-in Well-Temperature-Profile simulator," Journal of Petroleum Technology, September 1991, pp. 1140-46.
Kritikos, W.P., and Kutasov, I.M. "Two-Point Method for Determination of Undisturbed Reservoir Temperature," SPE Formation Evaluation, March 1988, pp. 222-26.
Kutasov, I.M., "Dimensionless Temperature, Cumulative Heat Flow and Heat Flow Rate for a Well With a Constant Bore-face Temperature," Geothermics, Vol. 16, 1987, pp. 487-72,
Taylor, A.E., and Judge, A.S., "Canadian Geothermal Data Collection-Northern Wells 1975," Earth Physics Branch, Energy, Mines, and Resources, Ottawa, 1976, No. 6, P. 43.
Ramey, H.J., Jr., "Wellbore Heat Transmission," journal of Petroleum Technology, April 1962, pp. 427-35.