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

Abstract
The 31S C/D Shale Reservoirs were producing at an annual exponential oil decline of 13%. Utilizing this decline, the oil production forecast for December 1995 would equate to 1132 BOPD However, with the application of an appropriate acid treatment based upon improved reservoir knowledge, the actual December 1995 production averaged 3821 BOPD. Furthermore, during the time period covered by this paper (FY 92 to FY 95), the 31S C/D Shale oil decline has been reversed to a current annual incline of 15%. This paper documents the efforts that went into formulating the highly successful acid treatment process.

Introduction
Twenty-nine remedial programs that included an acid stimulation as part of the procedure were completed during FY 1992 through FY 1995 in the 31S C/D Shale Reservoirs. This reservoir consists of laminated chert, porcelanite, dolomite, and shaly sand. The formation has low matrix permeability with small scale and microscopic fractures at orientations parallel and perpendicular to the bedding planes. This formation differs from other Monterey formations in that evidence of large widespread fracturing was not found in the 31S C/D shale.
The acid stimulations generated an average three-fold increase in production. The combined results from acid stimulations during the period of study have shown an increase from 643 to 2,337 BOPD In addition, the overall gas production has increased from 7,777 to 19,195 MCFPD while the average GOR decreased from 12. 1 to 8.21 MCF/BBL. The average water-cut also decreased from 80% to 69%.
The damage mechanisms being targeted are (1) drilling mud invasion; (2) solids precipitation; and (3) emulsion blocks. Core and well head samples (oil and water) were taken and analyzed for insights into damage mechanisms. Significant procedural changes to the acid program were made based on these analyses.

Drilling Mud Invasion
During the drilling process, mud cake forms to retard movement of the drilling fluids into the formation. This reservoir has small scale fractures which may not allow the filter cake to be formed as easily. Therefore, overbalance forces the drilling mud deeper into the formation. The production drawdown pressure to reverse flow and remove the damage is not sufficient and the zone remains partially plugged.
To analyze the drilling mud invasion, the initial production of each well was plotted against the corresponding overbalance pressure. (See Figure 1.) Although there are other factors such as different drilling/completion procedures, depth, and exposure time, there does appear to be a correlation on lower initial production with greater overbalance.

Solids Precipitation
A strong water influx is observed along the south flank with greater than 80% water-cut. The associated water production, combined with significant per well gas production has been observed to generate scale precipitation. Bailed samples of scale confirm these precipitates. A typical sample along the south flank have compounds of 93.6% CaCO3, 4.1% MgCO3, 2.3% FeCO3. A sample along the north flank have compounds of 48.5% CaCO3, 0.4% MgCO3, 51.1% FeCO3.

Emulsion Blocks
Emulsified oil was observed in surface samples taken at the well head from flowing wells. It is caused by the agitation and mixing of the produced oil, water, and gas during production. This process of emulsification occurs with the extraneous water from drilling mud, killing fluids and workover fluids, entering the formation.
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