Evaluation of Selective vs. Point-Source Perforating for Hydraulic Fracturing
Authors: Underwood P.J. , Kerley L. 
Published: December 2007

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This paper is a case history comparing and evaluating the effects of fracturing the Reef Ridge Diatomite formation in the Midway-Sunset Field, Kern County, California, using "select-fire" and "point-source" perforating completions. A description of the reservoir, production history, and fracturing techniques used leading up to this study is presented. Fracturing treatment analysis and production history matching were used to evaluate the reservoir and fracturing parameters for both completion types.

The work showed that single fractures were created with the point-source (PS) completions, and multiple fractures resulted from many of the select-fire (SF) completions. A good correlation was developed between productivity and the product of formation permeability, net fracture height, bottomhole pressure, and propped fracture length.

Results supported the continued development of 10 wells using the PS concept with a more efficient treatment design, resulting in substantial cost savings.

Introduction

Diatomite formations have low permeability with high porosity and require hydraulic fracturing to yield commercial production. Development of the Diatomite in this area of Midway-Sunset Field began in 1985, reaching peak production in 1988 at 1,900 BOPD (Figure 1). Less than anticipated production response from the 1994 program led to a review of the stimulation process. The review was to determine if the stimulation design could be modified to increase productivity or reduce treating cost.

Review of previous treatments using pressure history matching techniques indicated the presence of multiple fractures and near-wellbore tortuosity. This finding was supported by a reinterpretation of the initial treatments conducted in 1985-86. The severity of multiple fractures and tortuosity increases as the wellbore deviates from the preferred fracture plane. Minimizing the perforated interval minimizes the effects of multiple fractures. Since the authors believed that multiple fractures and tortuosity are primary causes of stimulation difficulties that can result in less production, they decided to develop a completion procedure to minimize these effects.

A 17-well drilling package scheduled for 1995 was used to evaluate the use of PS perforating and its effect on the stimulation process. Thirteen wells were modeled in this study, consisting of five PS wells and eight wells completed with the conventional SF perforating technique. Limited-entry perforating theory was used for both methods. Perforations were limited to 30 or fewer per stage to accommodate the designed 55-bbl/min limited entry injection rate.

For this project, a PS-perforated completion is defined as a perforated interval that is small relative to the larger pay section. The point source consisted of a 2-to-5 ft interval perforated with 2 to 6 shots per foot (spf) with 30 or fewer holes. While in theory, a single PS would fracture the whole interval; a total "leap of faith" to a single PS was not taken. The fear was that internal stress boundaries could limit fracture height and result in incomplete zone coverage. Typically, each PS zone was perforated with two PS intervals.

An SF-perforated completion consists of spacing 30 perforations evenly across the total interval to be fractured, avoiding casing collars and poor-quality zones.

Geology

Santa Fe Energy Resources has approximately one-quarter section of productive Diatomite on Section 9, Township 32 South, Range 23 East, Mount Diablo Base and Meridian, Kern County, California (Figure 2). The Upper Miocene, Reef Ridge, diatomaceous reservoir is found trapped below the Top Miocene unconformity. The surface of the unconformity strikes NW-SE and dips to the NE. P. 655