Ron Dusterhoft

How Minifracs Alter Leakoff and Ways to Counteract It

The analysis of minifracs in high-permeability formations has been the subject of several studies and debates. In many cases, the leakoff from the frac-pack appears to be less than that from the minifrac, which is normally conducted before the frac-pack. Consequently, the net pressure rise observed is less than the predicted net pressure based on the minifrac analysis. Rules of thumb have been developed to account for this apparent change in leakoff behavior. These rules include adjusting the spurt loss, choosing a latter-time closure press ire based on previous experience, and adjusting the observed fluid efficiency by an arbitrary factor. This paper attempts to provide a rationale for explaining the discrepancy between leakoff data collected from minifrac analysis and the observed frac-pack behavior. This paper also presents the results of laboratory experiments and field tests that show that the injection of a minifrac alters the observed fluid efficiency. Laboratcry tests evaluated the magnitude and character of the fluid loss from subsequent injections of a frac-pack fluid. These tests were conducted with cores ranging from 100- to 3000-md permeability. Field results from trials in which two minifracs were injected in the same zone are also presented. These field data are from wells in the Gulf of Mexico (GOM) and from a well in North Africa, all of which experience high fluid-leakoffrates. The results from these injections show that the effects of minifrac fluid can alter fluid efficiency significantly. Finally, the paper proposes methods and best practices for measuring or counteracting this effect by altering the sequence of injections nto the formation. Laboratory data supports the philosophy behind these best practices. In addition, field data illustrate the success of this altered injection schedule.