Re-Fracturing: Evaluation, Design, and Implementation of a Chester Oil Well in SW Kansas

Abstract

This paper will highlight the methodology used to successfully re-stimulate a well that had been unsuccessfully stimulated from previous treatments. A field example will present a well, which was previously treated on two occasions and show the subsequent implementation of a successful treatment that displaced both Ottawa and resin coated sand from the original fracture, enabling the third treatment to be successfully pumped. The initial evaluation methods that were utilized, which included pressure buildup (PBU), reciprocal productive index (RPI), and treatment pressure analyses, will show the damage that can occur when overly aggressive stimulation treatments are applied and present a methodology for evaluating a damaged well. A treatment was specifically designed to remove the proppant pack damage and create an effective stimulation for the character of the reservoir rock. The pressure response from the third treatment will demonstrate removal of the original and the creation of an effective stimulation. Post-treatment production data and analysis is also presented. Background The Gas Research Institute initiated a re-stimulation research project in late 1997 and early 1998. This project included both consulting companies and operators to research and implement re-stimulations on gas wells in selected study areas. The GRI presented a portion of the results in a one-day re-fracturing workshop1. Their presentation contained two primary parts. The first part considered various evaluation techniques to locate potentially economic re-stimulation candidates. The second presented case histories from operators who were working with the GRI contractors as part of this program. This paper is not concerned with reviewing their work, but simply to reference it and note key issues they raised to help provide context for this topic. Of interest now are some of the statistics that they presented in their introduction to the issue of re-stimulation. Re-fracturing treatments represent only 2–3% of the total amount of stimulation activity in the industry. This amounted to an estimated 450–550 re-stimulation treatments per year at the time of their presentation. It can be deduced that this low percentage results in reduced efficiency and a lack of familiarity with some of the unique challenges and issues that re-stimulation work presents. The GRI research stated that about 85% of all gas well re-stimulation activity is conducted in the Rockies, Mid-Continent and South Texas. This implies that this activity is primarily focused in hard rock environments and lower permeability reservoirs. They also stated that about 90% of the re-stimulation decisions were made without the benefit of reservoir analysis, but instead based upon production information alone. The GRI also was able to establish that there was a general negative attitude toward re-stimulation work in the industry. This paper discusses a re-stimulation case history. This particular project was chosen to facilitate the presentation of a systemic approach to re-stimulation projects by way of field example. In the case history well presented, an initial fracturing treatment had been conducted in 1995, and a subsequent treatment had been pumped in 1999. Both jobs screened out and production response was much less than expected. Following an evaluation of well data, a third treatment was successfully pumped in 2002. Evaluation, design, and implementation techniques were employed that should be applicable for many re-stimulation candidates in hard rock areas. Re-stimulation considerations Re-stimulation projects include three discrete steps. The initial step is to indentify candidates by analyzing in detail the fracture treatment and the resultant production performance. If production improvement is possible, the re-stimulation must be designed to remove the specific problems created by the initial treatment. Finally, on-site engineering is required to ensure appropriate fluid rheology design and to make any necessary real-time modifications to effectively implement the treatment.