Case History: 31S C/D Shale Reservoirs, Monterey Formation, Elk Hills Field, California
Authors: Hampton Thomas J. , Querin E. Mark , McCabe William D. 
Published: December 2007

Abstract
This is a field case history on increasing production from the 31S C/D Shale Reservoirs by 2,500 barrels of oil per day over a 3 year period. The 31S C/D reservoirs are shaly sand and siliceous shale intervals within the Miocene Stevens Zone.
The improvement in production was done by utilizing a team process covering five major activities:Development of a Geologic Model,Reservoir Modeling,Implementing the Gas Injection/Cycling Pilot,Improved Acid Stimulations, andInfill Wells.

Introduction
This poster session is a pictorial field case history on increasing production from the 31S C/D Shale Reservoirs by 2,500 barrels of oil per day over a 3 year period. The reservoir is located in the Naval Petroleum Reserve No, 1 (NPR-1) in Kern County. The 31S C/D reservoirs are shaly sand and siliceous shale intervals within the Miocene Stevens Zone of the 31S Structure. It contains a large secondary gas cap with a narrow oil band at the base of the steeply dipping beds, Historically, the reservoirs have been produced by depletion drive with well spacing at 10-40 acres. To conserve reservoir energy, the policy has been to shut in high gas oil ratio wells,
The improvement in production was done by utilizing a team process covering five major activities:

(1) Development of Geologic Model
A new geologic model was developed that explains C/D shale production trends, By the early 1990's, oil and gas production from the shale had significantly exceeded initial predictions. To help improve drilling and remedial decisions, a new geologic study was initiated. Several hundred feet of core from Well 315X-31S were obtained for reservoir rock and fracture system identification. In addition to traditional tests, confocal microscopy was conducted to identify microfractures and gas desorption analysis was conducted to identify unconventional gas sources.
Based on these studies, the C interval is interpreted as a finegrained distal turbidite unit with oil production from multiple thinly bedded sands interlayered with mudstone and shale. The most productive portion of the D interval consists of porous biogenic chert which has poor matrix permeability. The D is productive because of a network of interconnected micro-fractures. Large scale fractures, which are commonly reported from many Monterey reservoirs, are relatively minor in the productive intervals of the C/D shale.
Gas desorption analysis on core samples was conducted to evaluate additional sources of gas which cannot be accounted for in simulation models, These are believed to be the first desorption tests performed on Monterey formation shales. The desorption tests indicate an average yield of 88 SCF/ton. The extrapolated yield for the nearly 1 million acre-ft volume of the shale indicates a large gas volume which justifies further serious investigation.

(2) Reservoir Modeling
Segment simulation studies and economic sensitivity runs were made to evaluate and justify a gas injection pilot. A segment area was evaluated by a mechanistic black oil simulation, There were 20 by 20 cells for each of the 11 layers for a total of 4,400 grid cells (20 × 20 × 11). Gas injection was predicted to be profitable method for incremental oil recovery. While no single prediction is accurate in a mechanistic model, the trend indicated better recovery by gas injection.
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