Reasons for Production Decline in the Diatomite, Belridge Oil Field: A Rock Mechanics View
Authors: Strickland Frank G. 
Published: December 2007

Strickland, Frank G.; SPE; Dow Chemical U.S.A.

Summary
This paper summarizes research conducted on diatomite cores from the Belridge oil field in Kern County, CA. The study was undertaken to explain the rapid decline in oil production in diatomite wells by investigating three of six possible reasons. Characterization of the rock indicated that the rock was composed of principally amorphous opaline silica diatoms with only a trace of crystoballite quartz or chert quartz. Physical properties tests showed the diatomite to be of very low strength and plastic. It was established that long-term creep of diatomite into a propped fracture proceeds at a rate of approximately 1.5 microns/D [1.5 /d], a phenomenon that may contribute to rapid production declines. Also revealed was a matrix strength for the formation of about 1,325 psi [9136 kPa], a critical value to consider when depleting the reservoir. This also may help to explain the phase transformation to Opal CT around 2,000- to 2,500-ft [610- to 762-m] depth.

Introduction
The sample depths for this study were 1,530 and 1,915 m [466 and 584 m]. The study was undertaken to investigate the rapid decline in oil production in wells drilled in diatomite. The approach was two-fold:a characterization of the rock was taken andthe physical properties of the diatomite were investigated as they apply to fracture stimulation.
Before noting specifics, we discuss the reasons certain tests were chosen over others. The rapid production declines may be caused by one or all of at least six reasons.Erosion or abrasion of the soft formation face by hard, dense, angular proppant generates fines that later, during production, migrate and progressively seal off the proppant pack permeability(see Fig. 1).Initial embedment of the hard proppant into the soft surface of the formation immediately following the completion of the fracture treatment causes flaking of the surface. This generates particlesfree to migrate; in addition, the proppant pack width is decreased substantially (see Fig. 2).Long-term creep of the soft formation around the proppant is causedby high in-situ pressure and/or temperature encountered downhole. This also causes loss of the effective proppant pack width (seeFig. 3).As the fluids and/or gas are produced from the formation, the porepressure or reservoir pressure is decreased, which has the effectof increasing the effective stress (compressional load) on the proppant. When this value exceeds the strength of the formationface, large-scale embedment takes place with a resulting drop inpermeability and production.The intersection of a limited number of natural fractures during stimulation treatment caused flush production. As the fissures aredrained, the production declines to what can be produced throughthe tight matrix permeability.The formation has very low relative permeability to oil.
Of thesesix possibilities, only three (2,3, and 4) were addressed during this research. One (5) was supported by observations of what appeared to be natural fractures in the cores. Specimen preparation was far from being trivial. The soft nature of the diatomite led to disking, desiccation, and expansion upon retrieval from depth. Extreme care had to be taken in handling the core. It had to be sealed into a polyvinyl chloride (PVC) pipe with drilling mud poured into the annular space. In addition, the core was hand carried from the well location by plane to the laboratory, and specimens were prepared within a few days after arrival. Even with such precautions, the amount of usable samples obtained was just sufficient to conduct one each of the following tests, with no duplications at this time. Consequently, the results discussed further must be taken with caution, since no statistical theory could be applied at the time.

Geological Description
Issacs described three phases of mineralogy in the diatomite, with transitions occurring from temperature and pressure changes (Fig. 4). He mentioned that each type has significantly different properties, especially in the areas of porosity and permeability. Characterization of the rock was conducted using X-ray diffraction, spectrographic analysis, scanning electron microscope (SEM) analysis, petrographic microscope, and energy-dispersive analysis on the SEM.

Petrographic Microscope Analysis
Thin sections of the cores were prepared from each of the two zones and examined under the petrographic microscope.
JPT
P. 521^