Cement Sheath Stress Failure

Abstract

Summary Observation of the sudden appearance of annular pressure in wells exposed to high temperature changes or excessive internal casing pressure prompted a laboratory investigation to simulate conditions under which cement sheath failure could occur and thereby define the causes, characteristics, and limits of the problem. Cement sheath failure is manifested by interzonal problem. Cement sheath failure is manifested by interzonal annular-fluid movement and abnormally high annular pressure at some point behind the casing up to and at the surface. Cement sheath point behind the casing up to and at the surface. Cement sheath failure can be observed in any producing area where excessive flowing temperatures exist at the surface or where excessive internal casing test pressures are used. The detrimental effects of cement sheath failure are numerous and may include lost revenue from lost production, potentially hazardous rig operations (especially when annular isolation loss creates shallow-water sands supercharged with gas), and potentially hazardous producing operations. Exposure of steel casing to excessive temperature increases or internal test pressures causes diametrical and circumferential casing expansion. This circumferential force creates a shearing force at the cement/casing interface, causing failure at the cement/casing interface or radial fracturing of the cement sheath from the inner casing surface to the outer casing (or borehole) surface. Introduction In several operating areas, annular-flow problems not attributable to common annular-flow-after-cementing definitions are experienced. This paper is not intended to discuss short-term annular-influx problems. Long-term annular-influx problems usually experienced problems. Long-term annular-influx problems usually experienced after a well begins producing represent a completely different set of circumstances. Long-term annular influx generally occurs after excessive casing test pressures once the cement sheath has set and attained some compressive strength, or following excessive temperature changes resulting from excessively high producing temperatures or steam-injection temperatures. Long-term annular influx has long been believed to be caused by either cement sheath failure or hydrostatic pressure loss in a channeled (bypassed) mud column after the weighting material has settled out of the drilling mud. An extensive investigation was begun to determine the reasons for these long-term annular-flow phenomena. Analysis of cementing systems and well cementing techniques concentrated on the use of "good cementing practices" (i.e., pipe movement; effective casing centralization; sufficient circulation times and rates before cementing for mud and hole conditioning; and sufficient volumes of water, washes, or spacers for hole cleaning). Such current cement sheath evaluation devices as fluid-compensated bond logs or ultrasonic-type logging devices were used to determine the presence of primary cement channels. After analyzing only a relatively few problem wells, it became evident that something drastic had problem wells, it became evident that something drastic had happened to the cement sheath in each well. In all the wells investigated, clean cement was circulated to surface with no indication of lost circulation or fallback; however, the presence of a cement sheath was not evident. The only evidence of cement in the annulus visible on the bond logs was an approximate 50% decrease in amplitude; no evidence of casing or formation signal was visible on the microseismogram of the bond log. The presence of a cement sheath was extremely difficult to prove on the ultrasonic logs as well. Without evidence of a viable cement sheath on any of the logging devices, the existence of a mud channel in the primary cement sheath was difficult to ascertain. With the understanding that full circulation was attained during primary cement placement, gas-cut cement is readily identifiable on ultrasonic logging devices, cement particles cannot enter formation-matrix permeability, and cement (once it has set) does not magically disappear from the annulus, it was readily apparent parent that something had destroyed the cement sheath. Further investigations indicated that all these production casing strings had been exposed to either high internal test pressures or high surface flowing (or injection) temperatures.