Injection Profile Corrections-A Review Of Workover Techniques, Willard Unit
Authors: Downs S.L. , Gohel Manhar K. 
Published: December 2007

Of the many different methods that have been tried in an effort to improve the injection profiles of San Andres water injection wells in the Willard Unit, only onethe full inner-string recompletion using a limited-entry technique and super-thick fracturing fluids has consistently given good results.

Introduction

Wasson field, situated in Yoakum and Gaines Counties of West Texas, lies on the northern shelf of the Permian Basin (Fig. 1). The principal reservoir is Permian Basin (Fig. 1). The principal reservoir is the prolific San Andres of Permian age. The field has been formed into seven units or project areas, and waterflood projects now cover all the field. The Willard Unit lies in the north central area of the Wasson field. It covers 13,130 productive acres, all of which are under waterflood. Reservoir limits are controlled by structure, porosity pinchout, and a tilted water table. Thickness ranges from 230 ft in the southwest corner to less than 50 ft in the north. Original oil in place was 600 million STB. Average porosity is 8.5 percent and average permeability is porosity is 8.5 percent and average permeability is 1.5 md. The workovers described in this paper have all been in the south two-thirds of the unit where original oil in place averages 60,000 STB/acre. The San Andres formation at Wasson field was deposited on a broad, flat, shallow-water carbonate shelf over which the seas transgressed and regressed many times. The productive San Andres interval at Willard Unit is mostly subtidal. It consists of sediments that are calcareous particles, fragments, and grains mixed in various combinations, dolomitized and altered by diagenetic processes. Porosity and permeability have been controlled by the interaction permeability have been controlled by the interaction of leaching, dolomitization, emplacement of anhydrite, and the original depositional fabric. The thin-bedded primary lithologic framework and complex diagenetic history result in a layered reservoir. Good-porosity zones 1 to 20 ft thick are separated by low-porosity zones a few inches to a few feet thick. Both good- and low-porosity zones have wide geographic distribution relative to thickness and may usually be correlated several well distances. The low porosities are associated with absolute permeabilities porosities are associated with absolute permeabilities of 0.1 md or less. Laboratory-derived capillary pressure curves and log-analysis-derived water saturations pressure curves and log-analysis-derived water saturations indicate that these low-porosity zones have high water saturations. Oil/water relative permeabilities indicate that permeability to oil is nil and permeability to water is very low. The low-porosity layers present barriers to vertical flow and effectively maintain a preferred horizontal movement of fluids through the preferred horizontal movement of fluids through the reservoir. Fig. 2 shows results of a core analysis performed to evaluate the effective layering of the reservoir. The 180-ft interval shown represents a section of the reservoir from the top of the pay at 5,060 ft, through the water/oil transition zone, and 40 ft into the underlying aquifer. Porosity and vertical permeability are plotted vs depth. The analyses were performed on whole-core segments 4 in. in diameter and 3.3 to 6.7 in. in length. Permeabilities in the shaded intervals are 0.1 md or less; they are associated with low porosities and are considered to be barriers to vertical porosities and are considered to be barriers to vertical flow. Many of the wells currently in injection service were originally completed between 1937 and 1943 as producing wells. producing wells. JPT

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