Integrity of Downscaled Well Models Subject to Cooling

Abstract

The annular cement is often subject to temperature variations, for example heating during steam injection, or cooling during CO 2 injection or hydraulic fracturing. Fractures caused by thermal stresses may create flow paths through the cement sheath thereby jeopardizing the well integrity. The objective of this work was to experimentally and numerically investigate the effect of thermal cycles and harsh cooling on downscaled wellbore sections with a focus on the cement sheath failure.Downscaled wellbore sections were each prepared by cementing a casing section within a hollow cylinder of Berea sandstone. One specimen was subjected to a thermal cycling program that included cooling to Ϫ50°C and heating to 80°C. Two other specimens (dry and wet) were used in harsh cooling experiments with solid CO 2 and liquid nitrogen. CT scanning was performed before the onset of, during and after the completion of the thermal cycling program or harsh cooling experiments.No differences in the distribution of voids and fractures in the cement before and after the thermal cycling were observed. This indicates that the applied temperature range was not sufficient to cause damage in the cement sheath. On the other hand, the harsh cooling experiment using liquid nitrogen resulted in fracturing of both the cement and rock in the water-saturated specimen.Finite-element simulations reproducing the thermal cycling program used in the laboratory experiment were performed. The simulations have shown that thermal radial stresses at cement/casing and cement/ sandstone interfaces were relatively low, i.e., from 0.5 to 1.0 MPa, which might explain the lack of thermal-induced damage in CT images. The simulations also suggest that the radial fracture in the water-saturated specimen subject to harsh cooling was caused by a combined effect of tensile hoop stresses and stresses due to water freezing. The fracture was most likely initiated in the sandstone followed by further propagation into the cement. It is likely that well construction materials and the surrounding formation are water-saturated. Thus, cooling well below 0°C, which may occur during a CO 2 blowout, would increase chances for cement and formation failure.

Integrity Experiments for Geological Carbon Storage (GCS) in Depleted Hydrocarbon Reservoirs: Wellbore Components under Cyclic CO2 Injection Conditions

“…Overall, the results of temperature cycling presented here (casing-cement and cement-CT) conform with our previously published results [51,66]. Todorovic et al [42] conducted similar experiments on wet composites of casing-cement-rock, and they observed no change in cement structure using CT scanning when they applied cooling down to −40 °C on the composite. Roy et al [67] carried out experiments on the dry casing-…”

“…Roy et al [67] carried out experiments on the dry casing-cement-rock composite in the range of −50 • C-+80 • C and observed no change in cement structure using a CT scan as well. However, experimental studies by Todorovic et al [42] and Roy et al [67] were carried out without confining pressure, e.g., the worst-case scenario. We believe that effective permeability measurement is more reliable than a CT scan, where its results are resolution-dependent.…”

“…Several studies have been implemented on down-scaled composites to investigate the thermal cycling effect [40,41]. Nevertheless, these studies were limited to temperatures above 0 • C. The pioneering work of Todorovic et al [42] applied deep cooling on dry and wet casing-cement-rock composites. However, they used only computed tomography scans to evaluate cement integrity.…”

Digital Cement Integrity: A Methodology for 3D Visualization of Cracks and Microannuli in Well Cement

“…The experimental set-up and procedure to generate defects in the cement sheath have been described in detail in previous publications [41][42][43][44][45][46][47][48][49], and only a brief description will be given here. The experimental set-up consists of rock (usually sandstone), cement and casing to resemble a downscaled section of a well.…”

“…In our group, we have recently developed an experimental characterization procedure where we use CT for 3D digital visualization of well cement in realistic annular geometries. With our dedicated and tailor-built cement sheath integrity experimental set-up [41,42], we have visualized microannuli and cracks in cement formed during thermal cycling [41,[43][44][45][46] and pressure cycling [42,[47][48][49], and we have also used the digital cracks and microannuli obtained from CT scans as imported leak path geometries in computational fluid flow simulations [45,[50][51][52]. Furthermore, we have also used CT to study cement-formation bonding [53][54][55][56], cement durability [57,58], and cement-mud interactions [59].…”