Hydraulic Fracturing in Tight, Fissured Media

Abstract

Summary Large volumes of natural gas are found in tight, fissured reservoirs. Hydraulic fracturing can enhance recovery, but many complications, such aspressure-sensitive or accelerated leakoff, damage and complex fracturing, ariseduring treatment of such reservoirs. Special procedures generally should beconsidered during breakdown and procedures generally should be consideredduring breakdown and fracturing of these reservoirs. In addition, the use ofalternative stimulation strategies may be beneficial. Introduction Large quantities of natural gas exist in low-permeability reservoirs ofseveral western U.S. basins. The Natl. Petroleum Council identified greaterthan 600 Tcf [greater than 17 × 10–12 m3] of recoverable gas in both lenticularand blanket reservoirs. The U.S. Geologic Survey recently estimated that thereis 420 Tcf [12 × 10–12 m3] of gas in place in the Piceance basin alone and4,970 Tcf [141 × 10–12 m3] in the Greater Green Piceance basin alone and 4,970Tcf [141 × 10–12 m3] in the Greater Green River basin. If only a smallpercentage of this gas is recovered, these basins can provide significantreserves. Characteristics of these reservoirs make gas production difficult andgenerally uneconomic. Matrix rock permeabilities are typically less than 1 udunder in-situ conditions. Fissures, or natural fractures, exist in most ofthese reservoirs, but the fissures are often quite marginal, increasing theeffective permeability of the reservoir by only one to three orders ofmagnitude. Nevertheless, the presence of fissures is critical to the recoveryof gas from these rocks. These fracture systems also have often been found to be highly anisotropic, with most fissures aligned along a single axis. Permeability anisotropy iscaused by the preferred orientation of the fissure system and probably bystress anisotropy as well. Hydraulic fractures often are probably by stressanisotropy as well. Hydraulic fractures often are aligned with the fissures, soproductivity improvement ratios are low. Complex fracturing is likely in fissured media, owing to close alignment ofthe fissures and the hydraulic fracture and to the likelihood of developingoffsets when cross fissures are encountered. Such fracturing may result in hightreatment pressures, early screenouts, and significantly reduced fracturelengths. Enhanced leakoff is common in fissured reservoirs. Leakoff into the fissuresmay be quite complex because of a variety of complicating factors. The fissuresmay have a relatively constant conductivity during hydraulic fracturing, or theconductivity may be a function of the net stress on the fissure so that leakoffincreases during a treatment. In extreme cases, the fissures may begin todilate and leakoff accelerates rapidly. Dealing with these complications is particularly difficult in a tightreservoir because the fissures must be treated carefully in leakoff control orcomplex fracturing. The fissures are the production mechanism and must not bepermanently damaged or the reservoir will not produce. This paper discusses the complications associated with hydraulic fracturingin tight, fissured media and presents strategies for controlling or alleviatingsome of the deleterious effects. Some alternative technologies are alsosuggested. Much of this paper is based on results of the U.S. DOE Multi-well Experiment (MWX) and mineback experiments at the DOE's Nevada Test Site. Leakoff The most obvious effect that permeable fissures have on a hydraulic fracturetreatment is enhanced leakoff. If the leakoff is relatively insensitive topressure, as expected in fissures with a vuggy porosity, then it can bedetected with a minifracture and controlled with a suitable additive. However, the additive must not totally plug the fissures, or it must be easily cleanedout during flowback. In fissures with relatively planar porosity, such as nonmineralized cracks, the fissure aperture may be highly pressure-sensitive. During a hydraulicfracturing treatment, the pressure within the fissure increases as fluid leaksinto it, so the normal stress on the fissure decreases and its conductivityincreases. This change is somewhat compensated for by an increased normalstress caused by the opening of the hydraulic fracture, but the overall effectis a decrease in the net stress on the fissure and a pressure-dependentleakoff. Castillo developed analyses for pressure-dependent leakoff. Castillodeveloped analyses for pressure-sensitive leakoff where the leakoff coefficientis proportional to pressure-sensitive leakoff where the leakoff coefficient isproportional to pressure to the 0.5 or 1.0 power. Pressure behavior of astress-sensitive pressure to the 0.5 or 1.0 power. Pressure behavior of astress-sensitive fissure, however, is considerably more complex.