Recent Acid-Fracturing Practices on Strawn Formation in Terrell County, Texas

Abstract

The goal of an acid fracture treatment is to generate a highly conductive pathway of sufficient length from the reservoir to the wellbore. Depth of penetration of live acid is the critical factor in determining the success of an acid-fracturing treatment. Depth of penetration is controlled by the acid reaction rate, leakoff, and stimulation rate. Acid reaction rate is a function of several factors, the most important of which is the reservoir temperature. Yet another concern, in acid fracturing in long carbonate intervals, is attaining the necessary diversion to ensure that multiple sets of perforations are adequately stimulated. Because of their high solubility and highly fractured/vugular nature, carbonate reservoirs in the Permian Basin show excellent response to acid fracturing treatments. However, inadequate diversion can leave substantial portions of the reservoir untreated. Different acid systems have been developed to counter the problems in acid fracture stimulations. Chemical and mechanical means of diversion have been used with varying degrees of success. Likewise, there have been many attempts made in retarding the reaction rates of hydrochloric acids in high temperature environments. Recently, there has been a large number of highly successful acid fracture treatments in the Permian Basin incorporating a combination of new polymer-free self-diverting acid combined with an existing acid-oil emulsion technology. This paper will discuss a combination of technologies, which has recently been applied successfully in the Strawn formation in Terrell County, Texas. It will also focus on what is being done to mitigate the affect of high temperature on hydrochloric acid's reaction rate. It will further develop improvements in reservoir characterization and pay zone determination, which has been improved by the utilization of resistivity imaging logs. Some examples presented contain information from radioactive tracers and production logs, which are fundamental to understanding how good zonal coverage was achieved using different techniques. Additionally production analysis has been conducted to determine the effective fracture half-length and etched conductivity. Finally, a relative comparison between the old and new completion methodologies is made taking into account that the new completion practices have only been applied in full combination since 2004. Background The Permian Basin in West Texas, USA is renowned for its prolific carbonate reservoirs. Covering an area in excess of 86,000 square miles, the basin is both vast and diverse in reservoir types and qualities. Even within specific areas and reservoirs, the degree of heterogeneity is broad due to the depositional and diagenetic history of the basin. Reservoir heterogeneity complicates every aspect of a well's life, from drilling to completion. New technologies and methods have greatly enhanced production by offering data and solutions that were not available in previous years. Likewise, technologies often considered archaic by today's fast paced standards continue to pay dividends when properly integrated with the new technology and enhanced methodology, which is constantly developing. Although relatively new with the Permian Basin timeline, completions in the Strawn formation in Terrell County have endured just such an evolutionary process. Production from the Strawn Formation in Terrell County increased significantly in the 1990's with the discovery of the Abilene Christian University (ACU) Strawn and Deer Canyon Strawn fields. Subsequent development of the Strawn over the past decade has resulted in a better understanding of reservoir characteristics and improved completion techniques. This maturation has allowed better decision-making as well as more efficient recovery of reserves, both of which have become increasingly necessary as depletion occurs throughout the field.

Development of Acid Fracturing Model for Naturally Fractured Reservoirs

“…Then, alternating stages of acid and pad are injected followed by flush stage (Kalfayan 2001). The injection of intermittent stages of pad has a purpose of cooling down the formation to reduce the acid reaction rate and, as a result, increase the depth of acid penetration (Zaeff et al 2007). Also, it creates a filtercake on the fracture surface, which helps to decrease the acid loss into formation (Coulter et al 1976;Zaeff et al 2007).…”

“…The injection of intermittent stages of pad has a purpose of cooling down the formation to reduce the acid reaction rate and, as a result, increase the depth of acid penetration (Zaeff et al 2007). Also, it creates a filtercake on the fracture surface, which helps to decrease the acid loss into formation (Coulter et al 1976;Zaeff et al 2007). In addition to that, alternating injection of pad and acid promotes viscous fingering phenomenon that takes place inside the fracture when less viscous acid pushes more viscous pad fluid down the fracture (Kalfayan 2001).…”