Gas fracking and cement cracking By Jilda Rush • April 25, 2010
Everyone is familiar with cement. But have you ever thought about its use in gas wells? In the gas industry, cement plays a crucial role.First, the driller penetrates all the strata and the “cuttings” are removed, leaving a bare borehole. Next, a metal pipe called a casing is placed in the borehole. Most people believe fluids and gases can’t possibly get through the casing, therefore the casing will protect underground aquifers from contamination. Wrong. Pollution can still occur in the space between the outside of the casing and the inside of the borehole, called the annular space. This is why drillers force a cement mixture down the inside of the casing, then up the outside of the casing into the annular space until the cement fills this area and returns to the surface. This cement is the chief mechanism for protecting water sources from contaminants.
Having been an asphalt/concrete materials tester, I am concerned over the extreme conditions deep drilling operations will exert on this concrete. Portland Cement by nature is too brittle and low in tensile strength to withstand pressures and vibrations. A study, conducted by the petroleum industry itself and titled “From Mud to Cement-Building Gas Wells,” illustrates the results of improper cement selection and design. (Google this title to download the study.) “Since the earliest gas wells, uncontrolled migration of hydrocarbons to the surface has challenged the oil and gas industry. Gas migration can lead to sustained casing pressure (SCP). By the time a well is 15 years old, there is a 50 percent probability it will have measurable SCP in one or more of its casing annuli. However, SCP may be present in wells of any age. Cement damage can occur long after the well construction process. Even a flawless primary cement job can be damaged by rig operations occurring after the cement has set. The mechanical properties of the casing and the cement vary significantly; consequently, they do not behave in a uniform manner when exposed to changes in temperature and pressure. As the casing and cement expand and contract, the bond between the cement and casing may fail.”
Conventional cement is designed for optimal ease of placement and strength. Emphasis on strength at the expense of durability often leads to the development of SCP. Conventional cements also shrink during setting. Therefore, specially engineered cements can and should be designed which expand, thereby tightening the hydraulic seal; and, flex in unison with the casing rather than failing from tensile stresses.
Each gas well needs a cement designed for its specific geological attributes. However, this requires significant underground studies, then incorporating the data into computer simulated wellbore models so the cement can be subjected to the same pressures, temperatures, and chemicals actually encountered underground. Drillers rarely conduct these studies. State regulators and citizens should demand such studies to ensure proper cements are designed; especially since Pennsylvania Department of Environmental Protection proved that cement failure caused contamination of several water wells in Dimock. Remember, proper cement is the chief mechanism to protect water sources from contaminants.
Jilda Rush of Windsor is a former Oregon Department of Transportation Engineer.
Jilda Rush’s letter originally published at PressConnects Viewpoints: Gas Fracking and Cement Cracking