This work examines the variability in fluid leak-off rates measured under static and dynamic conditions. Laboratory generated data is compared to field measured data, and the conditions under which static and dynamic data should he used for frac design are examined.
The control of fluid leak-off in both the low permeability matrix and highly permeable natural fractures is examined on formation cores under a variety of conditions. An evaluation of the mechanism of control offered by various fluid loss additives is conducted by examining the fluid-matrix and fluid-filter cake interactions.
References and illustrations at end of paper.
Fluid leak-off during hydraulic fracturing can exceed 70% of the injected volume if not controlled properly. A consequence of high leak-off can be the severe curtailment of production due to formation matrix damage, adverse formation fluid interactions or simply altered fracture geometry. An overwhelming amount of fluid can be necessary 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 leak-off characteristics of a particular formation is essential in order to both select proper fluid loss control measures for the treatment and predict fracture geometry.
Advances in pressure analysis have made possible the estimation of formation fluid leak-off rates from pressure decline following injection1. However, this method depends upon a knowledge of gross fracture height; therefore, it is best applied in formations with a large net permeable height. It has recently been proposed that it is possible to estimate leak-off rates during pumping from changes in the frac gradient2.
The leak-off rates obtained from field measurements are important not only because they provide realistic numbers for the prediction of fracture geometry and job design, but they provide as well a yardstick for laboratory measurements of leak-off and the development of fluid loss control methods and additives.
When such field data is not available, laboratory Cw and spurt data are generated with the fluid in question on formation core samples. The conditions under which these tests are run can dictate the resulting leak-off 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 satisfactorily controlled, various dynamic methods of testing are compared, and the relationship of static and dynamic leak-off coefficients is discussed.
An attempt is made to correlate field measured leak-off data with laboratory results. Both field and laboratory data show the overriding influence of leak-off to natural fractures and/or high permeability streaks. Once the high leak-off to these areas is curbed, the leak-off to the low permeability matrix is strongly influenced 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 leak-off rates throughout the treatment. Available methods for incorporating variable leak-off rates into frac design programs are discussed.