Perforating for Stimulation: An Engineered Solution

Abstract

Summary Point-source perforating has reduced fracture-stimulation expenses in the south Texas Lobo trend. Although the producing intervals frequently exceed 100 ft, the operator typically perforates the bottom 5 ft of the interval. Data collected during perforating are used to design optimized fracture-stimulation treatments. The optimized treatments average 55% less proppant than the jobs designed previously with conventional practices that had perforations across the entire pay interval. The new well's initial production and sustained performance has been higher than conventionally stimulated wells of similar potential. In addition, the point-source-perforated wells have produced significantly less proppant than similar wells with perforations across the entire pay interval. Point-source perforating has reduced the cost of perforating, proppant, fluids, equipment rentals, and cleanup. This case-history paper presents the following.Historical account of previous completion practices (limited entry, multiple-stage fracturing, and full-zone perforating).Discussion of the "perforating for stimulation" fracturing technique currently used.Comparison of production results.Formation-evaluation methods (logs and bottomhole gauges).Perforating methods (phasing, charges, and perforating interval selection).Results from a recompletion confirming that depletion has occurred. Introduction Perforating is a key component to the completion of most wells. Typically, the decision made about where to perforate is based on the location and the extent of the pay. The process of perforating is usually limited to choosing the appropriate gun with the best performance and scheduling the job. In the overpressured, low permeability environment of south Texas, a new perforating philosophy has been adopted, in which "perforating for stimulation" is the practice instead of "perforating for production." This new concept places the emphasis on the best perforation method for stimulation and obtains key fracturing parameters during the perforating stage of the completion. Experience shows that commonly accepted perforating techniques can cause well-designed stimulation treatments to fail. Cost Savings The operator has drilled approximately 150 Lobo wells over the last 5 years in the south Texas counties of Webb and Zapata. The experience gained over this period has reduced completion costs by U.S. $125,000 per well. These savings are derived from an overall change in the stimulation philosophy and the ensuing procedural changes this new philosophy created. Lobo Geology The Lobo formation is a member of the lower Wilcox group and has produced approximately 8 trillion ft3 of gas from two south Texas counties. The Lobo is a low-permeability, geopressured sandstone that was subjected to massive slump faulting at the end of its deposition. This created abundant faulting within the Lobo trend to the degree that individual blocks were created, usually 80 acres or less. This is a significant factor because each well is its own reservoir. Table 1 lists the typical reservoir parameters. Initial Development Consolidated Oil and Gas discovered the Lobo trend in 1973 with the N.H. Clark No. 1.1 Because of the complex structural nature of the Lobo and poor fracturing techniques, its full potential was not immediately realized. A major geological breakthrough occurred with the introduction of 3D seismic in the early 1990s. Two-dimensional seismic was used previously but resulted in a 72% success rate. With the introduction of 3D seismic, the success rate improved to 92%. Also, improvements in fracturing equipment, breakers, completion procedures, and 3D fracturing models have been significant factors in making the Lobo trend economic. Past Practices In the past, completion strategy was based on perforating every productive foot of pay derived from log analysis. Attempts to stimulate entire intervals were accomplished with large pad volumes, low Ottawa sand concentrations, and limited use of effective breakers. After the treatment, the wells were shut in overnight to allow the bottomhole temperature to break the thick, crosslinked gels. In later years, it was realized that all perforated layers were not being stimulated. The use of limited-entry perforating became popular in an attempt to stimulate the entire perforated section. This method limited the number of perforations to create excess perforation-friction pressure. In theory, this would increase the bottomhole treating pressure to greater than the pressure necessary to break down higher-stress zones, thus distributing the stimulation treatment to all perforated horizons simultaneously. The shortfalls of both these methods were recognized when cumulative production did not match predrill volumetric estimations. The next method involved limiting the treatment interval to one of the three primary Lobo sands and performing individual treatments on each sand package. This method showed improvements but still lacked the optimum results. Observations and a New Concept The factors leading to the new concept of "perforating for stimulation" were conceived from production-log results, the lack of success in adding perforations, tracer surveys, and information provided in the advance-stimulation technology work done by Aud, Wright, Holditch, and others.2 In an effort to better understand the completion effectiveness, production logs were run on a selected group of wells. The most revealing information was that the majority of the production usually came from a small interval, which was the best pay. Fig. 1 shows a production log from a hydraulically fractured well with the entire 100-ft interval perforated. More than 90% of the 1,800 Mcf/D was entering from only a 5-ft interval.

"Super Height Growth" —Strange Surface Pressures and Refrac Potential

“…Similar behavior was described by Lestz [1]. In a Wilcox zone in a different area (as seen in the log sections in Figure 9) wells were typically perforated as seen for Well A -with several lobes of sand perforated and then a large (on the order of 500,000 lb of 16-30 RC sand) fracture treatment pumped.…”

Screenout Detection and Avoidance

“…Perforating a small interval minimizes the creation of multiple fractures and SO events. Lestz et al [35] give more details.…”

Perforating and Hydraulic Proppant Fracturing in Western Siberia, Russia

“…Only more recent studies have shown that perforating for hydraulic fracturing is not the same as perforating for production. 10,[12][13][14][15] The following paragraphs provide a quick overview of the most important criteria for perforating for hydraulic fracturing.…”

“…The limited-entry perforation technique became popular and was used to keep the number of perforations to a reasonable limit. 2,13 The idea was that the limited number of perforations would create excess perforation-friction pressure, which, in theory, would increase the bottomhole treating pressure to larger than the pressure necessary to break down higher-stress zones, thus distributing the fracturing treatment to all perforated layers. This technique was successful in some areas.…”