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Summary

This work examines the variability in fluid leakoff rates measured under static and dynamic conditions. Laboratory-generated- data are compared to field-measured data, and the conditions under which static and dynamic data should be used for fracture design are examined. Control of fluid leakoff in both the low-permeability matrix and highly permeable natural fractures is examined on formation cores under a variety of conditions. The control mechanism offered by various fluid-loss additives is evaluated by examining the fluid-matrix and fluid-filter-cake interactions.

Introduction

Fluid leakoff during hydraulic fracturing can exceed 70% of the injected volume if not controlled properly. A consequence of high leakoff can be the severe curtailment of production because of formation matrix damage, adverse formation fluid interactions, or altered fracture geometry. An overwhelming amount of fluid can be required to achieve a desired fracture geometry in a massive hydraulic fracturing treatment; thus, fluid efficiency can govern the economic success of the treatment. A knowledge of the leakoff characteristics of a particular formation is essential both to select proper fluid particular formation is essential both to select proper fluid loss control measures for the treatment and to predict fracture geometry. Advances in pressure analysis have made possible the estimation of formation fluid-leakoff rates from pressure decline following injection. The method depends on a knowledge of gross fracture height; therefore, the method is best applied in formations with a large net permeable height. Using changes in fracture gradient to permeable height. Using changes in fracture gradient to estimate leakoff rates during pumping has been proposed recently. Leakoff rates obtained from field measurements are important not only because they provide realistic numbers for the prediction of fracture geometry and job design, but also because they provide a yardstick for laboratory measurements of leakoff and the development of fluid-loss control methods and additives. When such field data are not available, laboratory wall-building coefficient (C.) and spurt data are generated by applying the fluid in question to formation core samples. The conditions under which these tests are run can dictate the resulting leakoff coefficient. Apart from efforts to simulate actual pumping conditions, wide variations can result from simple static testing procedures. In the work described herein, several factors affecting the outcome of fluid-loss tests have been identified. Once those factors are controlled satisfactorily, various dynamic methods of testing are compared. An attempt is made to correlate field-measured leakoff data with laboratory results. Both field and laboratory data show the overriding influence of leakoff to natural fractures and/or high-permeability streaks. Once the high leakoff to these areas is curbed, leakoff to the low-permeability matrix is influenced strongly by the shear rate within the fracture, particularly in high-rate jobs in the early part of the pad where fracture widths are relatively small. This can lead to changing leakoff rates throughout the treatment. To account for variations in C,, during a treatment, Crawford 4 suggests multiplying laboratory values by a factor of . Characteristics of a particular formation can dictate the fluid-loss additive or combination of additives required to achieve necessary leakoff control. Particulate additives are essential in controlling leakoff to high-permeability streaks or formations, while liquid hydrocarbon additives function well in low-permeability matrix control. The interaction of each of these additives with the formation and filter cake affects their performance. This work looks in detail at the interaction of hydrocarbon with complexed fluids and low-permeability formations.

Discussion

The leakoff rate during a hydraulic fracturing treatment has a marked effect on the final geometry of the fracture. With higher-efficiency fluids, less fluid is required to achieve a desired fracture length. For example, when the fracture is contained within zone, decreasing C., from 0.001 to 0.0001 ft/min 1/2 [0.03 to 0.003 cm/min 1/2] increases the efficiency from 30 to 90%, and makes it possible to use one-third as much fluid to achieve a desired possible to use one-third as much fluid to achieve a desired fracture length (Fig. 1). Using the same example, increasing fracturing fluid efficiency also increases closure time (Fig. 2). Ninety-percent fluid efficiencies alone become impractical, since it is desirable for the fracture to close on the proppant before gel degradation to minimize proppant settling. In a large number of fracture treatments, where the fluid stability and/or break rate is tailored to 12 to 24 hours, fluid efficiencies of no greater than 70% are advisable.

JPT

P. 1071