Fracturing Results in Diatomaceous Earth Formations, South Belridge Field, California

Abstract

Summary Our company began fracturing diatomaceous earth zones in the San Joaquin Valley (CA) in 1976. Fracturing has proved an effective method of exploiting these previously noncommercial reservoirs. Nevertheless, productivity behavior is typified by high initial rates followed by rapid decline. Reasons for this decline have been evaluated and are discussed. Also discussed are laboratory experiments performed to determine an appropriate fracture design for this formation. Introduction In 1976 Mobil fractured a well in a diatomaceous earth formation in the San Joaquin Valley. This was a highly successful treatment. Since then, hundreds of fracture treatments have been performed in several fields in formations of this type in the valley. This paper summarizes our hydraulic fracturing experience in the South Belridge field from early 1977 through early 1980 with particular emphasis on evaluation of the production behavior of these wells. Background At the time our research began, fracture treatments were uncommon in the diatomaceous earth zones in the San Joaquin Valley. Very modest, discouraging results were reported from one area1 with somewhat more encouraging results being reported from another area2 when oil-base fracturing fluids were used. Personal communications with various sources confirmed that fracturing had met with very limited success in these formations, and yet the evidence from limited core and production data indicated good potential for production improvement through fracturing. Therefore, we began a research and testing program to determine the fracturing potential of the diatomaceous earth zones in South Belridge field. Because few data were available on fracturability of these soft rocks and on the potential damage by aqueous fracturing fluids, we first performed some laboratory experiments on core samples. The initial fracturing designs were based on the results of these experiments. Reservoir Description Diatomaceous earth deposits in the South Belridge field form the bulk of the diatomite and brown shale zones of the Reef Ridge formation. This has been the primary formation of interest in the fracturing work in this area. Some portions of the upper part of the McLure chert also have been fractured. Depths of the productive intervals vary somewhat because of structural position but overall are between 800 and 3,000 ft [244 and 914 m] from ground level. These formations are Miocene and lower Pliocene in age. They are characterized by high porosity (50 to 70%), low permeability (generally less than 10 md to air and much less than that to oil in situ), and moderate to high (40 to 80%) water saturations. Pore size distribution (PSD) measurements indicate pore sizes in the micron to submicron range. Photomicrographs confirm this, showing whole and fragmented diatom tests of this size. This fine-grain character gives a core sample the appearance of shale even though the major component is amorphous silica. Clay minerals are present, reaching as high as 25 wt%. Of this, ±80% is mixed-layer illite and smectite. In general, clay content increases with depth. Zone quality appears generally to decrease with depth. Log and core analyses give values for porosity, oil saturation, and permeability, which all generally decrease with depth. There are some exceptions to this. Fracturing Considerations and Laboratory Evaluation Fracturing operations in the South Belridge field introduced some rather unconventional fracturing problems and concepts. The unusual mechanical properties of diatomaceous earth materials raise questions about fracturability and fracture closure problems. Wells in the South Belridge field are closely spaced, and oil-producing intervals are scattered throughout very thick zones. Individual oil-rich zones are difficult to identify from well logs. On the basis of the best available log analysis, it appeared most practical to complete a large portion of the entire upper part of the diatomaceous earth zone. This called for a series of fractures generated in such a way as to produce maximum fracture height but only modest length. As a practical goal, we hoped to produce individual fractures of 200 to 300 ft [61 to 91 m] in height and lengths of the same order.