Abstract
Hydraulic fracturing operation is a common method to improve the production of unconventional reservoirs. Pressure cycles created by alternating loading and unloading procedures of hydraulic fracturing is a crucial factor impacting cement sheath integrity. At high pressure condition, cement exceeds the elastic limitation and behaves more plasticity. The objective of this study is to assess the stress distribution of cement sheath for evaluation of the development of failure and evaluation the effect of internal casing pressure cycle on the cement integrity. A 2D finite element model was employed to simulate casing-cement-casing systems. Brittle and ductile cement systems were compared. The formation and casing were simulated as elastic materials while the cement was simulated using elastoplastic properties based on the lab test data. Alternating high and low casing pressure resulted by loading and unloading procedure was applied on the inner surface of casing. Radial and hoop stress were analyzed to determine the formation of failure. Plastic strain was shown to illustrate the plastic deformation within the cement sheath. The results showed ductile cement had better performance than brittle cement. After pressure cycle, residual hoop stresses in brittle cement were notably higher than ductile cement indicating higher risk of failure in subsequent pressure cycles. Residual radial stresses were observed to be closer in magnitude for both cements. Plasticity had influence on the risk and location of failures. Location of potential failure moved away from casing-cement interface in both cements as internal casing pressure increased.
In the last decades, hydraulic fracturing has been an efficient and widespread stimulation technique to improve the production of wells. Based on the report (Independent Petroleum Association of America 2008), 90% of oil and gas wells in the U.S. used hydraulic fracturing for well stimulation. The initial flow rate can be increased 1.5 to 30 times, as well as the overall production from 5 to 15%. At the meantime, hydraulic fracturing injects millions of gallon of water into oil and gas containing formations, the induced seismic activities are becoming the major concern. Cyclic hydraulic fracturing is employed alternating high and low injection rate to balance the production improvement and environmental issues (i.e. reduce induced seismicity). The cyclic injection rate results high and low pressurization to trigger cement failure (i.e. micro-annulus) and casing failures in the wellbore.
