Summary Hydraulic fracturing is widely used within the petroleum industry to stimulate production rates of wells. The effectiveness of these stimulation treatments depends on the selection of the proper materials to maximize the stimulation effect. Once proper materials are selected, use of substandard materials on site in the hydraulic fracture treatment can reduce the effectiveness of the treatment. We developed a procedure for field monitoring and for evaluating the quality of the materials used in stimulation treatments. The program consists of on-site quality-control checks for water quality, gelled fracturing fluids, proppants, and other additives commonly used in stimulation treatments. A field kit and manual support the program. Often, these on-site quality-control procedures reveal deficiencies that were corrected before a treatment was initiated. If these deficiencies had not been corrected, the treatments may have prematurely terminated and/or the stimulation that resulted from the prematurely terminated and/or the stimulation that resulted from the treatment may have been diminished. Introduction An analysis of service company charges for acidizing and fracturing treatments performed in 1983 indicated that bidding resulted in a 39% savings in the cost of stimulation treatments when compared with list prices. Realization of the magnitude of savings achieved from bidding generated some concern regarding the quality of the product being delivered to location. Chevron's West Texas Div. product being delivered to location. Chevron's West Texas Div. requested and funded the development of a fracturing quality-control program with an overall goal to achieve savings through bidding program with an overall goal to achieve savings through bidding without sacrificing quality. Our well stimulation treatments are generally designed by production engineers. Service companies are then asked to bid on the production engineers. Service companies are then asked to bid on the treatments based on these designs. This process of developing our own treatment designs significantly improved results of stimulation treatments and allowed uniform bidding by service companies. Each service company bids on the same type and amounts of fluids, proppants, hydraulic horsepower, etc. proppants, hydraulic horsepower, etc. To remove further bias from the bidding process and to ensure that a satisfactory product is delivered at the well site, a quality-control program was developed. The objectives of this quality control program areto maximize the productivities of wells that are stimulated,to inform the service companies of management's interest in obtaining quality service and materials,to evaluate service companies fairly so that substandard performance can be eliminated and superior performance encouraged,to obtain feedback from the service companies on how to improve quality of well treatments, andto increase our knowledge of the fracturing process. A viable quality-control program requires adequate evaluation of all stages involved in the treatment. For fracturing treatments, this often begins with the primary cementing program and can include openhole logging, perforating, perforation breakdown, fracturing-fluid tank inspection, water-source evaluation, proppant supplier evaluation, proppant evaluation on location, and evaluation of the service company equipment and personnel performance during the treatment. The cementing, openhole logging, and perforating programs are beyond the scope of this paper. Proper quality control requires an additional engineer whose sole responsibility is to monitor materials, equipment, and personnel performance for the entire job. The effort to obtain improved quality performance for the entire job. The effort to obtain improved quality increases treatment costs, but these costs are usually insignificant compared with the increased productivities of the wells over their producing lives. By having an engineer with the quality-control producing lives. By having an engineer with the quality-control re-sponsibility on location, the other engineer monitoring the job is free to evaluate pressure and rate responses during the job properly and to make any changes in the job procedure dictated by the formation's behavior during the fracturing process. Quality control is a cooperative effort between the service company and the production company designed to improve continually or to upgrade the stimulation service provided on location. Through this cooperative effort, problems can be identified and solutions formulated to enhance overall stimulation success.

Currently, a major part of our oil and natural gas production comes from wells in tight naturally fractured or fissured formations. In many cases, the commercial viability of these wells depends on the success of hydraulic fracturing. Properly designed fracture treatment enhances the production from these wells substantially. However, the complexities of fracture treatment of such reservoirs often lead to premature screenouts due to accelerated leakoff in the fissures from stress sensitivity. Normally, the fissures dilate with increased treatment pressure and the permeability of these fissures increases exponentially as suggested by Walsh1 model for stress sensitive fissures. Such stress or pressure sensitivity of fissures need to be diagnosed and remedied before a successful optimum treatment can be achieved in these formations. Conventional minifrac analysis of injection pressure and decline pressure often helps in diagnosis of such pressure dependent leakoff case. The original Nolte decline analysis was for wall building fluids or for a constant leakoff case. When this analysis is applied to the pressure dependent leakoff case, the fluid efficiency is over-predicted suggesting inadequate pad volume and pump rate requirements. Such designs with inadequate pad volumes often cause proppant dehydration and premature screenout. This paper presents a modification to Nolte's decline curve analysis that helps in the diagnosis and evaluation of pressure dependent leakoff. It is also observed that such analysis leads to substantially lower fluid efficiency calculation resulting in increased pad volume and treatment rate. By applying the suggested method of analysis an empirical relation between the net pressure and fluid leakoff coefficient can be obtained.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThe manuscript will outline limited entry design techniques for the simultaneous fracture stimulation of the Fruitland Coal and Pictured Cliffs sandstone in the T28N -R7W Federal Unit of the San Juan basin, Rio Arriba County, New Mexico. The discussion will focus on the initial perforation design, stimulation parameters, flowback and post audit analysis. Brief discussion on operational issues and cost savings resulting from these techniques will be presented.Geologically, the Fruitland Coal above the Pictured Cliffs formation may be separated by as little as three feet of shale in the T28N -R7W Federal Unit. It is possible when stimulating the lower formation that the shale is not a 'fracture barrier' and communication with the upper formation, the Fruitland Coal, can be established. In these cases, because of zonal communication, reserves from the Fruitland Coal cannot be accessed.Utilizing limited entry techniques and incorporating stress and leakoff data from offset wells, both formations have simultaneously been fracture stimulated effectively. This methodology allows accessing reserves in the Fruitland Coal that may otherwise not be produced. Radioactive tracer and production logs will be presented as supporting evidence to validate the successful well completion design and stimulation of both formations. Subsequent completion design changes resulting from the log review will be discussed.

This manuscript will outline the stimulation design criteria for wells in the Lobo Trend in south Texas and fracture design changes based on post fracture production analysis using commercial software. Production data will be modeled with the corresponding fracture geometry. Model results will be used as the benchmark to discuss fracture design changes and the effect on well performance. Case histories will focus on tight gas sands with reservoir quality ranging from 0.05 to 1.5 md with some pressure depletion. Multiphase flow effects in areas with higher water production will be investigated. Some discussion will focus on the details of calibrating fracture model inputs using open hole logs, radioactive surveys, sonic logs, production logging tools and mini-frac analysis. Results comparing post fracture well production, utilizing a pseudo 3D fracture design model and industry available production matching models, will show the effects of permeability, pressure depletion and multiphase flow effects on well performance. Understanding this information will allow the stimulation design engineer to better predict how geometry and fracture conductivity will alter well performance under varying reservoir conditions. With this knowledge, the engineer can better design ‘fit for purpose’ treatments, including those that may require extreme design changes in order to improve gas reserve recovery. Introduction The Wilcox (Lobo) trend in Webb and Zapata counties in south Texas is a series of geopressured, low permeability sands with an average depth from 5,000 to 12,000 ft. The Wilcox (Lobo) section consists of a sequence of stacked Paleocene age sands and shales overlain by the Lower Wilcox shale of Eocene Age. Extensive faulting, present in the Lobo section, has resulted in a slump complex of rotated fault blocks. The Lobo trend extends from Webb and Zapata counties to the south and west into Mexico. Permeability ranges from less than 0.1 md to 1.5 md. Figure 1 shows the location of the Lobo fields adjacent to the Mexico-USA border in south Texas.1 As an operator in this prolific gas producing area in south Texas, implementing effective hydraulic fracture treatments is a requirement in order for Conoco to economically produce the low permeability sands in the Lobo trend. This paper describes the engineering activities that were part of the development of a process to design and implement a pseudo three-dimensional (P3-D) fracture-modeling program in the Lobo trend in south Texas. The technique of hydraulically fracturing a formation to increase production rates and available reserves to commercial levels is a common practice within the petroleum industry. From industry surveys in the 1990's, approximately 56% of the wells drilled in the various geographic areas of the United States of America require fracture stimulation.2 The placement of a conductive fracture in the producing sands requires an optimal proppant fracture design process and proper field execution of the design. The engineering tools available for proppant fracture design have evolved during the past two decades. In the early 1980's, two primary mathematical models were developed and refined for modeling the complex hydraulic fracturing process. One model developed by Khristianovich and Zheltov3 incorporates the assumption of a rectangular shape in the vertical cross section of the fracture. A second model developed by Perkins and Kern4 and modified by Nordgren5 incorporates the assumption of an elliptical shape in the vertical cross section of the fracture. These two-dimensional (2-D) fracture models that assume a constant height vertical fracture have been widely used in the petroleum industry. Comparisons of these 2-D fracture models6 indicate that both models adequately agree with field data and that both models need to take into account changes in instantaneous shut-in pressure during treatments.

This paper will evaluate the efficiencies of completion methods in a South Texas field utilizing the latest techniques in post fracture production analysis.Stimulation effectiveness for each frac stage in ten multi-zone wells is evaluated. Effective values for reservoir and fracture parameters including porosity, permeability, propped fracture half-length, fracture conductivity and fracture face skin will be derived using production analysis techniques and will be compared for the different completion methods employed.The holistic model will incorporate the geological, petro-physical properties of the formation and production logging data. Actual stimulation and production data from ten wells in the same area are used in this analysis. Five of the wells were completed in single-stage fracture stimulation across multiple perforated intervals.Five wells were completed with two-stage fracture stimulations across multiple perforated intervals.The multiple layer fracturing technique was utilized in all wells. The study will derive the effective reservoir and fracture parameters using production allocation for each interval in the multiple interval wells. This paper will compare the different completion techniques using this methodology and will discuss a predictive model for future stimulation work in this area.This methodology will also help in identifying under-stimulated zones in existing wells that may be candidates for re-fracturing. Introduction The wells included in the study are part of the Wilcox Lobo Trend located in Zapata County in South Texas.The Wilcox (Lobo) trend in Webb and Zapata counties is a series of geopressured, low permeability sands with an average depth from 5,000 to 12,000 ft (1,525 to 3,660 m).The Lobo section consists of a sequence of stacked Paleocene age sands and shales overlain by the Lower Wilcox shale of Eocene Age.Extensive faulting, present in the Lobo section, has resulted in a slump complex of rotated fault blocks.The Lobo trend extends from Webb and Zapata counties to the south and west into Mexico (Figure 1).Effective permeabilities are less than 0.1 md.Implementing an effective hydraulic fracture treatment and an evaluation process for stimulation effectiveness are requirements to economically produce the low permeability sands in the Lobo trend. The wells presented here are nearby offset wells in the Lobo field (Figure 2). These wells were completed in 2003.The target intervals in these wells are primarily three zones.All the wells were fractured with similar fracturing fluids, intermediate strength proppants and aggressive breaker schedules utilizing multiple layer fracturing techniques. Background Extensive work has been conducted around fracture treatment design and evaluation of wells with multiple zones with most of the work focused on the use of limited entry techniques to effectively place proppant across multiple zones [1,2,3,4].The limited entry technique utilizes perforation friction to divert designed fluid and proppant volumes into multiple zones.This method is utilized when the economics do not justify multiple stages or when multiple stages cannot be placed effectively[5].This technique has been successfully implemented in the Lobo field in numerous wells.The success comes from applying the formation evaluation and log analysis into a fracture modeling process, and from the use of limited entry design guidelines [6,7].Tracer surveys and production logs were obtained after numerous stimulation treatments to develop these guidelines.However, tracer surveys provide an estimate of the fracture height and production logs provide contributions from each zone in a snapshot of time.