Page 166 - Basic Well Log Analysis for Geologist

LOG INTERPRETATION CASE STUDIES

Case Study 2
Mississippian Mission Canyon Formation

Williston Basin

An Ordovician Red River wildcat well is in progress in Canyon Porosity zone (Fig. 66). Consequently, you decide
the Williston basin. As drilling proceeds, two zones—the to begin your evaluation of the Mission Canyon by
Mississippian Mission Canyon and Devonian developing a Pickett crossplot which will give you a quick
Duperow-—are encountered which have hydrocarbon analysis of the distribution of the different water
shows. The well has just penetrated the Devonian Duperow, saturations, Values from the log (Fig. 62) for neutron
the second “show” zone. A decision is made to stop and porosity (,) and from the log (Fig. 61) for deep resistivity
drill stem test (DST) the Duperow. Unfortunately, the drill (R,..*), when crossplotted on two-by-three cycle log-log
stem test tool fails and becomes firmly stuck. Several paper, will show these water saturation distributions.
attempts are made to remove the tool but none succeed. In order to establish an R, line in the Pickett crossplot.
Meanwhile, wall collapse (caving) begins to develop within you use the following information:
the well bore.
R, =Ry, x F
Now you face the choice of either halting drilling with the
hope that the first show zone, the Mississippian Mission Where 10% porosity, F = 1/2 or 100
Canyon Formation, might be a productive reservoir or, of
R, = 0.023 x 100
continuing efforts to remove the DST tool. If the fish (the
stuck DST tool) ts successfully removed from the hole, the or
Duperow can be tested and drilling continued to the Red
R, = 2.3 ohm/meters
River Formation. However, continued attempts to recover G
the fish will undoubtedly lead to further deterioration of the The slope of the R, line equals 2 (average slope for
hole, and logging measurements taken when hole carbonates).
conditions are poor, may be unreliable. Also, it is possible Choose any five points (depths) in the following Mission
the DST teol cannot be dislodged. Canyon Log Evaluation Table (work Table B) and calculate
Because of the exploratory nature of the well, you made a the values for the blank spaces (complete the Table for those
decision several weeks ago to core the Mission Canyon at a five depths).
depth of 9.302 to 9,358 ft. Twenty-five feet of oil-stained, After determining values for the different Jog parameters,
fractured. microcrystalline dolomite was recovered, and the you can use the formula for volumetric calculation of
remaining core consisted of microcrystalline limestone and recoverable oil to evaluate the productive potential of the
anhydrite. After coring. se eral DST’s were tried and when Mississippian Mission Canyon Formation. The formula for
nene were successful, the well was drilled ahead. volumetric calculation of recoverable oil ts:
You decide the information from coring is favorable
7758 x DA x h& @ * Sy X RF
enough to halt drilling, to log to the top of the fish while
ut BOI
hole conditions appear still reasonably good, and then to
assess the potential of the Mission Canyon. Your company Parameters used in your calculations of recoverable oil are:
agrees with your suggestion that a very complete log drainage area (DA) = 150-acres; reservoir thickness (h) =
package be run in the hole, since the well is exploratory and 28 ft: porosity () = 11%: water saturation (S,,) = 33.5%;
carbonates can often be harder to evaluate than sandstones. recovery factor = 20%; and reservoir barrels per stock tank
The following data are available to you: (1) resistivity of barrel (BOI) = 1.35. When the equation is solved for N,
the formation water (Ry) at Ty = 0.023; (2) resistivity of the the resulting value for N, represents stock tank barrels. A
higher water saturations in the lower part of the Mission Evaluation Table (work Table B) has already been partly
stock tank barrel is oil recovered at the surface after
mud filtrate (R,,-) at T; = 0.017; (3) temperature of the
shrinkage has occurred as gas separates.
formation (T,) = 207°F; (4) At, = 44.4 wsec/ft, which is
The Mississippian Mission Canyon Formation Log
the interval transit time for a dolomite matrix (Table 6); (5)
At, = 185 yesec/ft, which is the interval transit time of fluid
completed for you. Fill in the rest of the table as you
for saltwater mud (Chapter IV); (6) pmy = 2.82 gm/ce,
proceed with your calculations.
which is the matrix density for dolomite (Table 7): (7) pp =
1.1 gm/cc. which is the fluid density for saltwater mud
(Chapter IV); and (8) the surface temperature = 60°F.
Preliminary examination of a Cyberlook* Log reveals