Although unstimulated horizontal wells have been very successful in naturally fractured reservoirs and in reservoirs with gas or water coning problems, there are certain situations where fracturing a horizontal well may be a viable option. Because of the dependence of fracture orientation on well direction with respect to the stress field, the possibility of fracturing a horizontal well must be considered before the well is drilled. The appropriate contingency plans should be made to anticipate the possible low production from an unstimulated well. It should be remembered that fracturing a horizontal well may also dictate how the wellbore may be completed and/or oriented. Fracturing a horizontal well may take place in one of the following situations.Restricted vertical flow caused by low vertical permeability or the presence of shale streaksLow formation productivity due to low formation permeabilityThe presence of natural fractures in a direction different from that of induced fractures. Thus Induced fractures have a high chance of intercepting the natural fractures, orLow stress contrast between the pay zone and the surrounding layers. In this case, a large fracturing treatment of a vertical well would not be an acceptable option since the fracture would grow in height as well as length. Drilling a horizontal well and creating either several transverse or longitudinal fractures would allow rapid depletion of the reservoir through the fracture(s). Although fundamentally similar to fracturing vertical wells, fracturing horizontal has its unique aspects that require very special attention to secure successful treatment. Differences between horizontal and vertical wells exist in areas of rock mechanics, reservoir engineering, and operational aspects. To start with, the basic rock mechanics aspects are examined. Stress Distribution around a Horizontal Well The stress distribution around a horizontal well follows the same equations used for vertical wells. The equations for the radial, tangential and vertical stress for a horizontal are given by the following equations.1Equations 1 through 6 Fracturing Failure Criteria for Transverse Fractures Hubert and Willis2 failure criterion is commonly used to predict the breakdown pressure of vertical well. In vertical wells, fractures are usually axial and consequently the failure criterion is when the tangential pressure is less than zero. In other words, the tensile breakdown pressure under this failure criterion is given by the following equation.Equation 7 One may apply the same criterion to horizontal wells using equations 2 and 5, depending on the relative value of stresses. Since vertical stress is usually the largest of the three stresses, applying Hubert and Willis failure criterion to horizontal wells (equation 2) yields the following equation.Equation 8 It has been observed in the laboratory1,3 that Hubert and Willis failure criterion underestimates the breakdown pressure for a transverse fracture. This is basically because this failure criterion assumes the creation of an axial fracture due to a tensile failure of the rock. This criterion is valid for a vertical well or a horizontal well where the fracture is longitudinal. However it does not fit the situation where the fracture is transverse.

Summary Although unstimulated horizontal wells have been successful in naturally fractured reservoirs and in reservoirs with gas- or water-coning problems, fracturing a horizontal well may be a viable option in certain situations. Because of the dependence of fracture orientation on well direction with respect to the stress field, the possibility of fracturing a horizontal well must be considered before the well is drilled. The appropriate contingency plans should be made to anticipate the possible low production from an unstimulated well. It should be remembered that fracturing a horizontal well may also dictate how the wellbore may be completed or oriented. Fracturing a horizontal well may take place in one of the following situations:Restricted vertical flow caused by low vertical permeability or the presence of shale streaks.Low formation productivity because of low formation permeability.The presence of natural fractures in a direction different from that of induced fractures. Thus, induced fractures have a high chance of intercepting the natural fractures.Low stress contrast between the pay zone and the surrounding layers. In this case, a large fracturing treatment of a vertical well would not be an acceptable option because the fracture would grow in height as well as length. Drilling a horizontal well and creating either several transverse or longitudinal fractures would allow rapid depletion of the reservoir through the fractures. Although fundamentally similar to fracturing vertical wells, fracturing horizontal wells has its unique aspects that require special attention to secure successful treatment. Differences between horizontal and vertical wells exist in areas of rock mechanics, reservoir engineering, and operational aspects. To start with, the basic rock-mechanics aspects are examined.

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