An increasing number of gas wells that exhibit a large difference in reservoir pressure and fracture treating pressure recently have become candidates for hydraulic fracturing. When these reservoir conditions are encountered, the result is an abnormally high pressure differential driving the fluid leakoff mechanism during the hydraulic fracturing treatment. In some deep well applications, fluid loss differential pressures (FLDP) up to 10,000 psi have been witnessed.

Neither laboratory core testing nor conventional minifrac techniques have accurately predicted fluid loss behavior for fracturing treatments in these wells. Standard laboratory test procedures use differential pressures of 1,000 psi (as per API recommended test procedures) to determine fluid loss coefficients. Minifrac test methods and analysis techniques currently used for field application estimates of fluid loss parameters were modeled after data measured in gas reservoirs where normal fluid loss differential pressures occur.

Laboratory fluid loss testing typically has predicted significantly less fluid loss than actually is encountered. Use of this data in computerized fracturing simulators to design the treatment has resulted in many premature screenouts. Conversely, minifrac tests tend to be pessimistic and overpredict the fluid loss in wells with very high FLDP's. A number of treatments have been completed successfully following minifrac tests that predicted screenouts for the jobs actually pumped.

This paper reports on laboratory fluid loss tests conducted at differential pressures of up to 9,000 psi to determine the nature of the fluid loss occurring in these wells. Significant spurt volumes have been observed even on cores of less than 1 md permeability with fluid systems that do not exhibit measurable spurt loss at the conventional 1,000 psi test pressure differential. Also, the fluid loss coefficients (Cw values) have varied little with escalating pressure differential, contrary to many published correlations.1  Results from actual field cases also are reviewed as illustrations where the observed FLDP was several thousand psi during fracturing.

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