Abstract

Texaco has been conducting screenless tip-screenout fracture to stimulate production in the Kern River field of California. During 1998–2000, more than 500 fracture treatments were pumped in some 200 wells. These treatments caused average oil production to nearly triple. This paper documents the results of the tip-screenout fracture treatments conducted over the past three years and presents a statistical analysis to determine how the production response is related to the treatment parameters. The goal was to identify the conditions that on average lead to the optimum production response.

Introduction

The Kern River Field, located five miles northeast of Bakersfield, encompasses about 10,000 acres and holds approximately 3.5 billion barrels of oil in place. It is the fourth largest oil field in the United States in terms of original oil in place. The structure of the Kern River Field is a simple homocline dipping 4° to the southwest. The formation designated as the Kern River Series, at depths between 100–1,400 ft, is comprised of an alternating sequence of unconsolidated sands with considerable interbedded silts and clays. The sands characteristically exhibit 30% porosity and 500–3,000 md permeability containing crude with an average viscosity of 4,000 cp at reservoir temperature.1–3 Fig. 1 shows a typical log of the Kern River Series Formation. The pertinent reservoir data are given in Table 1.

Texaco operates about 5,800 producing wells in the Kern River Field. Most of the wells are drilled on 21/2 acre spacing with inverted nine-spot patterns, and have 7" casing cemented to TD. The wells are perforated with jet charges at four shots per foot with all oil zones open. The wells are produced using cyclic steam flooding. Due to the high-permeability and unconsolidated nature of this formation, many wells experience severe damage problems associated with drilling and completion fluids and formation silts and fines produced with the heavy crude. Although some efforts were made to find the causes of the damage, no consistent solutions have been obtained. Ironically, the drilling and completion fluids claimed to be non-damaging on some wells turn out to be causes of formation damage in neighboring wells. Several common stimulation methods were tried, including pressure washing the perforations with opposed cups, steam stimulation and various chemical washes. None was effective.

A pilot series of screenless tip-screenout fractures was started in the Kern River field in 1997. The treatments were primarily aimed at bypassing formation damage to increase well productivity. For this purpose, strong efforts were made in each stage of treatment to achieve tip screen out, so that sufficient fracture conductivity could be obtained. Control of fines migration and particle flow is a by-product of the treatment. Jones and Soler have provided detailed descriptions of treatment philosophy and well response for the early stimulation program.4 Due to the great improvement in oil production, the stimulation program was extended throughout 2000. By the end of 2000, more than 500 fracturing jobs had been pumped in some 200 wells, resulting in nearly triple the average oil production prior to stimulation. Fig. 2 presents a normalized average oil production from the first 159 wells treated. Over the past three years, the treatment design and implementation has been changed significantly in order to minimize the treatment cost and optimize production increase.

The purpose of this study is to determine how the production response is related to the individual treatment parameters including volumes of pad and slurry, proppant mass, pumping rate, tip-screenout time and net pressure buildup. The goal is to identify the conditions that on the average lead to the optimum production response from the viewpoints of stimulation and economics.

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