Optimizing Hydraulic Fracture Design in the Diatomite Formation, Lost Hills Field
Authors: Nelson D.G. , Klins M.A. , Manrique J.F. , Dozier G.C. , Minner W.A. 
Published: December 2007

Optimizing Hydraulic Fracture Design in the Diatomite Formation, Lost Hills Field D.G. Nelson, SPE, and M.A. Klins, SPE, Chevron U.S.A. Production Company, J.F. Manrique, SPE, and G.C. Dozier, Dowell Schlumberger, and W.A. Minner, SPE, Pinnacle Technologies

Since 1988, over 1.3 billion pounds of proppant have been placed in the Lost Hills Field of Kern County, California in over 2700 hydraulic fracture treatments involving investments of about $150 million. In 1995, systematic reevaluation of the standard, field trial-based fracture design began. Reservoir, geomechanical, and hydraulic fracture characterization; production and fracture modeling; sensitivity analysis; and field test results were integrated to optimize designs with regard to proppant volume, proppant ramps, and perforating strategy.

The results support a reduction in proppant volume from 2500 to 1700 lb/ft which will save about $50,000 per well, totalling over $3 million per year. Vertical coverage was found to be a key component of fracture quality which could be optimized by eliminating perforations from lower stress intervals, reducing the total number of perforations, and reducing peak slurry loading from 16 to 12 ppa. A relationship between variations in lithology, pore pressure, and stress was observed. Point-source perforating strategies were investigated and variable multiple fracture behavior was observed.

The discussed approach has application in areas where stresses are variable; pay zones are thick; hydraulic fracture design is based primarily on empirical, trial-and-error field test results; and effective, robust predictive models involving real-data feedback have not been incorporated into the design improvement process.

Introduction

A large part of California's oil production occurs in several San Joaquin Valley giant fields that have heavy oil (high viscosity) or low permeability. These fields remain highly oil saturated, several with over 1 billion barrels of remaining oil in place, even though they were discovered and initially developed in the early 1900's. Thermal EOR, waterflooding, and/or hydraulic fracturing techniques are now applied to these fields.

Lost Hills is one such field, was discovered in 1910, remains largely undepleted, and is currently near all time high production rates of about 33,000 bopd because of a combination of low permeability hydraulic fracturing, waterflood, and heavy oil thermal development. Beginning in the late 1980's, improvements in hydraulic fracturing design and implementation yielded results which justified initiation of an aggressive diatomite primary development well program on 2-1/2 acre spacing. Since then, diatomite development has progressed at the rate of about 60 wells per year.

As with any finite, natural resource, the Lost Hills field reservoir quality varies. Future 2-1/2 acre primary development wells will have poorer economic results, eventually failing to compete internally for capital. In order to delay this inevitable result, a "systems management" approach as advocated by Deming was used to optimize hydraulic fracture design. The first step involved gaining an appreciation of the interrelated nature of file reservoir and geomechanical system. This required a conceptual review of the geology, past development activity, and current stress state. Secondly, characterization of reservoir and hydraulic fracture quality was improved with special measurements. Modeling and prediction followed to identify opportunities and optimize designs. Finally, field testing was used to validate the predictions.

The Interrelated Reservoir/Geomechanical System Proper optimization of hydraulic fracture design in the Lost Hills diatomite formation requires an understanding of a complex multi-component, integrated system. Some aspects of the system such as large-scale geologic structure are relatively static and easy to measure and predict.

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