Page 175 - Basic Well Log Analysis for Geologist

LOG INTERPRETATION CASE STUDIES

are from lower porosity Zones in the Mission Canyon In order to support your decision to avoid perforating the
interval. These data points above very fine-grained are lower zones, you construct a bulk volume water: BYW)
probably also above irreducible water saturation (S,, ;,,) and. crossplot (Fig. 77). On the bulk volume water plot. data
therefore. cannot be used for determining grain size. points above 0.035 are from lower porosity zones which are
At this juncture in your log evaluation of the not at irreducible water saturation, and so these zones will
Mississippian Mission Canyon from 9,308 to 9.415 ft, you produce some water.
are optimistic about the productive potential of the well. On The position of data points on a bulk volume water
logs, the interval shows invasion, it has intermittent crossplot can indicate changes in types of carbonate
permeable and porous zones, and its rock type—dolomite— porosity. Points which are below 0.035 represent zones with
is usualy a good reservoir rock. But, you are concerned vuggy porosity. along with intererystalline porosity (Table
about whether or not completion should be attempted from 8).
the lower zones of the interval, especially from 9,370 to Even though you have examined crossplots of relative
9.415 ft. The Cyberlook? Log examination, the diminishing permeabilities which gave you information about the
separation of the resistivity curves with increasing depth, relationship between fluids in porous zones, you want more
and the high water saturations on the Pickett crossplot all specific information about each zones’s permeability. This
strongly support your judgement that production from the information is provided by a permeability plot of S,, i, vs. b
lower porosity zones in the Mission Canyon interval will (Fig. 78). Most of the data points plot with permeability
not be water-free. You don‘t know, though, how much values which are considered favorable in your area. Values
water these zones will produce relative to oil. range from 0.1 to over [00 millidarcies, but generally
Because you are primarily concerned about the water indicate a good reservoir.
saturations in the lower Mission Canyon interval, you One of your last log evaluation procedures is finding
continue vour log evaluation by comparing the relative values for the moveable hydrocarbon index (S,/S,,). for
permeability to water (K,,.). relative permeability to oil moveable oil saturation (MOS), and for residual oi]
(K,,), and percent water-cut. saturation (ROS). The moveable hydrocarbon index value is
Relative permeabilities to water (K,,.) of different zones less than 0.7, and so the oil is moveable. Oil moveability is
are Shown on a crossplot of Sy j,7 vs. Sy (Fig. 74). Data also apparent from the high moveable oil saturation and low
points, clustering on or below the zero permeability to water residual oil saturation values.
line, represent zones from which water-free production can Your log evaluation of this particular well has been
be expected. Data points above the zero line represent zones unusually complete. The extensive evaluation has, in part.

which will produce some water: the amount of water been necessitated by the exploratory nature of the well and
produced will increase as the points are further away from also by the water problems presented in the lower porosity
the zero K,, line. zones of the Mississippian Mission Canyon Formation.
Relative permeabilities to oil (K,,) of different zones are Furthermore. because of the log package used in the well, a
shown on acrossplot of S wirr VS. Sy (Fig. 75). Data points, large amount of data was available for analysis.
clustering around the 100% (K,, = 1.0) line, represent It was apparent rather early in the log evaluation process
zones which should produce 100% oil. Data points, with that the data seemed to support a decision to set pipe.
increasing distance from the 100% line, indicate zones Nevertheless, it was important to know the correct interval
which will produce increasing amounts of water. for perforating so that water production could be kept as low
The relative permeability to water (K,y) and oil (K,,) as possible.
plots illustrate that some of the zones in the Mission Canyon The estimated oil recovery from the Mission Canyon
will produce water. However, neither plot gives information Formation for a gross interval of 9,308 to 9,357 ft is
about the aaount of water cach zone will produce. To 353,110 stock tank barrels (STB). This oil recovery figure
determine the amount or percent of water which can be is based on the following parameters: drainage area = [50
expected from each zone, you construct a water-cut acres: reservoir thickness = 28 ft; porosity = | 1%: water
saturation = 33.5%; recovery factor = 20%; and BOI
crossplot.
+Remember to use the formula Swine = W F/2.000 in crossplots of: Kro. Krw. Kreg. clean-up, the well potential was 569 barrels of oi! per day
(estimated) = 1.35.
The water-cut crossplot (Fig. 76) reveals a percent
The Mission Canyon Formation was selectively
water-cut variation from 0 to a high of 50%. The percent of
perforated from 9,308 to 9.357 ft. After a light acid
water produced. however, shouldn't exceed 30% and will
generally be less than 15%¢. Higher water-cut values are
from lower porosity zones: therefore, the lower porosity
(BOPD), 3! barrels of water a day (BWPD), and 700.000
cubic feet of gas per day (700 mefgpd) with a gas/oil ratio of
zones should not be perforated.
1.230/1. During the first five months, the well produced
56,495 barrels of oi] and 5,802 barrels of water.
160