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Summary Producing natural gas from shale-gas reservoirs presents a great challenge to petroleum engineers owing to the low-permeability nature of this type of gas reservoir. Large-scale and expensive hydraulic-fracturing operations are often required for enhancing gas well productivity. Because of the shaly characteristics of the reservoir rock, the hydraulically fractured gas wells are vulnerable to damage by fracturing fluids. However, the true significance of the formation damage in shale-gas reservoirs is still not clear. It is highly desirable to have a simple method for predicting the degree of fracture-face matrix damage and for optimizing fracturing treatments. This paper is meant to fill this gap. A new mathematical model was developed in this study to predict the effect of fracture-face matrix damage on the productivity of fractured gas wells in shale-gas reservoirs. A unique feature of the new model is that it considers reservoir/fracture crossflow in finite-conductivity fractures. Results of the model analyses were sensitized to reservoir properties and facture-face matrix-skin properties determined by the fracturing-fluid properties and treatment conditions. Large ranges of possible leakoff and spurt-loss coefficients were investigated. We concluded that, in the ranges of reservoir and fluid properties used in this study, well productivity should drop by less than 15% even if the residual matrix permeability is reduced to only 5% of the virgin reservoir permeability in the damage zone. Neglecting the resistance to flow in the fracture will overestimate the effect of matrix damage on well productivity. The well-productivity drop caused by matrix damage is most sensitive to the invasion depth and damaged permeability.

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Fracture propagation mechanisms in coalbed methane (CBM) reservoirs are very complex due to the development of the internal cleat system. In this paper, the characteristics of initiation and propagation of hydraulic fractures in coal specimens at different angles between the face cleat and the maximum horizontal principal stress were investigated with hydraulic fracturing tests. The results indicate that the interactions between the hydraulic fractures and the cleat system have a major effect on fracture networks. ''Step-like'' fractures were formed in most experiments due to the existence of discontinuous butt cleats. The hydraulic fractures were more likely to divert or propagate along the butt cleat with an increase in the angles and a decrease in the horizontal principal stress difference. An increase in the injection rate and a decrease in the fracturing fluid viscosity were more conducive to fracture networks. In addition, the influence on fracture propagation of the residual coal fines in the wellbore was also studied. The existence of coal fines was an obstacle in fracturing, and no effective connection can be formed between fractures. The experimental investigation revealed the fracture propagation mechanisms and can provide guidance for hydraulic fracturing design of CBM reservoirs.

There are often plenty of horizontal planes of weakness in reservoir formations, especially in shale formations, as reported for a number of oilfields. Once the weak-plane fails, the formation will become unstable, and can easily undertake slippage across a large area along its interface. The number of casing failure caused by slippage of weak-plane has been increasing significantly in recent years. Wells with casing failure are concentrated in an increasing number of areas. However, there has been lack of research efforts on how to optimize cementing and completing parameters in order to prevent casing failure induced by formation slippage. To address the problem, a more advantage completing type has been elected by qualitative analysis. The calculation model of critical slip displacement in un-cemented conditions was established. A finite model was used to test and verify the analysis and the model. The critical slip displacement of casing shear damage was also calculated. In this study, a new cementing practice was then proposed by optimizing casing parameters according to API standards, and a new research method was also put forward by proposing new casing materials to effectively mitigate casing failure caused by formation slippage for the future. Modeling results indicate casing failure induced by formation slip is different from conventional casing damage. The slip displacement needs to be used to measure casing impairment inside of maximum stress. Casing elongation is the key parameter for controlling casing shear failure. The type that keep the weak-plane un-cemented exhibits a larger critical slippage displacement .So the casing with lower grade and smaller thickness is recommended in weak-plane if the casing could meet all other down-hole requirements. The new concept is very different from the common belief that the good quality cement and higher grade and thicker casing are safer. If the elongation of casing can be improved by 60%, the critical casing failure slippage displacement can be increased by 21.40%. In this study, a new casing design and well completion method to prevent casing failure caused by formation slippage was proposed, and some guidance was provided for manufacturing casing with new material that can effectively mitigate or delay casing damage.

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