Summary A new, fully integrated model of hydraulic fracturing can be used to compare measured and calculated pressures for parameter determination during the fracture treatment to improve the prediction of hydraulic fracture geometry. Sensor data obtained during the course of the treatment - such as wellhead flow rates, fracturing-fluid viscosity, and proppant concentration - are received directly by this model, which takes into account all the essential physical phenomena that influence the pressure response associated with hydraulic fracture growth. At any point during the treatment, the model can be rerun, faster than real time, changing reservoir and treatment parameters until the difference between calculated and measured wellhead or bottomhole pressure (BHP) histories is minimized. Updated predictions of final propped fracture geometry can then be made by running the model faster than real time, using the remaining treatment scheduled as input and adopting the parameters that correspond to the best history match; these predictions may differ substantially from that of the job design, thus providing field personnel with an improved estimate of the final fracture geometry before the treatment is completed, while remedies can still be implemented. Results show history matches of the pressure response and associated fracture geometries for three treatments performed in the Travis Peak formation of east Texas and one performed on a coal seam in the Piceance Creek basin near Collbran, in western Colorado. In addition, this work is contrasted with previous efforts to deduce fracture geometry from pressure-response profiles. Introduction A great number and variety of hydraulic fracturing models have been developed over the past 3 decades.1–7 Many have been applied in various ways to the design and analysis of treatments carried out on a commercial basis by the industry. Nonetheless, routine optimization of hydraulic fracture treatments - namely, the achievement of the greatest production possible for the smallest investment - remains a very elusive and desirable goal. Shortcomings in several areas have hindered the improvement of the fracturing process:the models used for design and analysis in many cases lack adequate descriptions of the physical phenomena;reservoir characteristics and fracturing-fluid rheology, which strongly influence fracture geometry, are often unknown or uncertain;inadequate monitoring of the fracturing treatment may diminish quality control; andinformation that becomes available during the course of the treatment is generally not used for updating the design prediction of final fracture geometry, which is usually generated from limited prefracture information. It is certainly true that substantial progress has been made over the past decade or so: fracture modeling has evolved from the early specialized constant-height formulations (Khristianovich-Geertsma-de Klerk1,2 and Perkins-Kern-Nordgren3,4) to the more general, fully three-dimensional (3D) simulators,5–7 as described in Ref. 8; minifracture and microfracture tests are occasionally performed to determine reservoir characteristics, such as in-situ stress distributions and the extent of fluid leakoff9–11; service companies have updated their on-site monitoring capabilities to ensure better execution of treatment designs11,12; and certain aspects of fracture creation (e.g., fracture-height containment) are determined from the pressure response during the treatment.9 These efforts have doubtless produced some improvements in treatment effectiveness. But even the most modern comprehensive simulators cannot accurately predict fracture geometry if pertinent reservoir characteristics are unknown; nor can the extent of fracture containment be inferred from the pressure response if rheological changes and sand staging are not taken into account. The optimization of hydraulic fracture treatments, therefore, requires substantive across-the-board improvements in which all the relevant capabilities in fracture modeling, data acquisition and interpretation, and field operations are synthesized into one coordinated system. Therefore, we propose a comprehensive methodology to improve hydraulic fracture prediction and to provide the basis for intelligent decision-making during the treatment. This methodology essentially involves detailing monitoring and real-time simulation and analysis of the fracturing process.13 It is based on the premise that the actual treatment record, and the information inferred primarily from the pressure response during the treatment, can be used to improve estimates of fracture geometry significantly over those derived from prefracture data and schedules. The essential aspects of this methodology can be summarized as follows.Detailed treatment monitoring (e.g., pressures, flows, rheology, and sand scheduling) and accounting of deviations from job design.Use of monitored sensor data as input to real-time hydraulic fracture models.Determination of unknown reservoir/treatment parameters and affirmation of known quantities by the history matching of observed and predicted response pressures.Best estimation of current fracture geometry using prefracture information, the history-matched parameters, and the real-time data flows.Updated predictions of future job status based on the best current fracture estimates and the remaining (or alternative) treatment schedule.Identification of any treatment pathologies and recommendation of possible remedies. Central to the analysis of the fracturing process during treatment is the real-time model of hydraulic fracturing and the history-matching procedure to determine unknown parameters. A summary of the fracture model is provided; its technical details are thoroughly described in Ref. 8. The major purpose of this paper is to describe the history-matching procedures and to show comparisons with actual field data, emphasizing the application of the model to the improvement of field fracturing operations.

In a major advance over previous capabilities for prefracturing design and real-time field application of hydraulic fracturing models, the ability to accurately handle complex pumping schedules and reservoir conditions has been achieved by developing the capabilities for full three-dimensional hydraulic fracture analysis and combining these with the speed of lumped model simulations. These lumped (spatially integrated) models have already been used for the design and realtime analysis of numerous fracture treatments. Now, a fully 3-D code, based on finite elements and surface integrals, developed as part of a comprehensive field monitoring and analysis project for the Gas Research Institute, is being used to analyze fracture geometries and pressure distributions for many variations of viscosity scheduling and stress barrier placement. Analysis of these 3-D results produces integration coefficients which can be implemented in the lumped models. These coefficients enable the lumped model to accurately account for viscosity and pressure variations in the fracture, and for differences in confining stress magnitude in the strata above and below the payzone. This work allows the lumped models to be made more quantitatively correct by incorporating the essence of full 3-D modelling capabilities without any loss of execution speed. The effectiveness of the methodology is demonstrated by performing simulations with a number of data sets from field operations as input.

A simplified simulation study of an actual software development effort is presented. A model is developed and exercised through various stages of modifications to an originally unreliable soft ware design until viable software design results. Techniques in model development, simulation, analysis, and language capability which lead to enhanced software reliability are discussed. Uniquenesses in the approach presented are contrasted to simulation methods which lack this capability.

1A rugged, very compact microcomputer, with a co-processor, has been developed for real-time, high-speed monitoring and simulation of field operations. The development of the computer was funded by the Gas Research Institute for supervision of hydraulic fracturing of gas and oil wells. The system handles data acquisition, data basing,. running of process simulator codes, and extensive operator Interface.The computer Is a fully compatible PC clone and uses a passive backplane architecture for easy repair and maintenance. Two Independent processors are used: a 4.77-MHz Intel 8088-and a 25-MHz Motorola 68020 32-bit Microprocessor. This reliable low cost, high performance system brings the power of large minicomputers such as a VAX 11 /780 into the field. Data acquisition is performed by a built-in board which enables sampling 16 (expandable to 256) channels of analog data directly from standard field Instrumentation. Alternatively, connection can be made to most service company vans, with isolation filters preventing Intrusion Into their electronics. Serial data can also be transferred from an Independent data acquisition computer. Acquired data Is stored In the data base where it is accessible to process simulation codes during the current Job and for comparison on future jobs.The Integrated 3-Dimenslonal Fracture Models running on the system are the most comprehensive simulations developed for real-time operation. Rapid, repeated running of the process models during the early stages of ·the job permits best estimates of uncertain reservoir parameters to be obtained. This real-time "history matching" is facilitated by extensive on-line help facilities, a totally menu-driven user interface, and high-resolution, high-speed dynamic graphics capabilities. These features also Improve operator awareness of current Job status and add significantly to the quality control.The system has been tested on dozens of field operations. It has been selected for use by a maJor oil company and Is under consideration by several other maJors.References and Illustrations at end of paper. 55