Treatment Diagnostics and Net Pressure Analysis Assist with Fracture Strategy Evaluation in the Belridge Diatomite
Authors: Minner W.A. , Wright C.A. , Dobie C.A. 
Published: December 2007

A wide variety of hydraulic fracture strategies are utilized by producers of Southern San Joaquin Valley diatomite formations. In most cases, producers have developed a standardized fracture design over the last five to ten years, which is applied on a mass scale in several large field developments. The rock is very forgiving from a fracture design and proppant placement standpoint; this forgiving nature belies the extreme complexity of diatomite fracture behavior. Most producers have an idea or estimate of the created fracture geometry. However, for the most part, these geometry estimates are not well-founded from the standpoints of fracture pressure analysis or reservoir production response.

This paper presents a case study of fourteen fracture stages in four wells, located in a closely spaced (5/8 acre) Belridge Diatomite waterflood. In this project, Crutcher-Tufts chose to evaluate a new completion strategy with shorter wells, fewer fracture stages, and a limited interval (point source) perforation technique. Twelve minifrac injections and fourteen propped fracture stages were evaluated using a combination of real-data (net pressure) fracture analysis and surface tiltmeter mapping. The minifrac injections were independently performed and evaluated before the propped fracture treatments, to identify stages with unacceptable fracture reorientation, and to compare these orientations with the larger volume propped frac injections.

Based on unacceptable minifrac orientation and marginal petrophysical log properties, one propped fracture stage was not performed. Even with the point source (limited interval) perforation strategy, the simultaneous growth of competing multiple fractures was required to explain observed fracture net pressure levels. Tiltmeter fracture mapping to showed multiple fracture growth in the form of vertical and horizontal fracture components. Several treatment behaviors were observed that are believed related to whether fracture growth was centered in the perforation interval or whether the induced fractures grew predominantly upward or (downward) away from the perforated interval. Average propped fracture half-length for the moderate-sized treatments pumped was estimated to be about 100 feet. With variations in fracture height growth behavior, the "point source"' perforation strategy is believed to have resulted in somewhat uneven, but reasonable height coverage of the target diatomite interval. Well production response supports this interpretation. In future wells, a multiple point source, limited-entry completion strategy may prove to obtain better, interval coverage.

Introduction

Hydraulic fracturing stimulation is routinely performed on a large scale to enable production from Southern San Joaquin Valley diatomite formations. Net pay thickness is often greater than 1000 feet requiring from 4 to 7 independent fracture treatment stages per well. Although diatomite porosity is high (30-70%), permeability is low (0.1.5 ml) unstressed air), and the low Young's modulus of the rock (50,000-250,000 psi) makes achievable fracture length short, Thus, well drainage radius is small, leading to field development well spacing from 2-1/2 acres to as small as 5/8 acre. To date, more than 1000 diatomite wells have been completed in several large oilfields, such as Belridge and Lost Hills; it is likely that an equal or greater number of future potential infill sites remain.