Experimental Laboratory Setup for Visualization and Quantification of Cement-Sheath Integrity

Abstract

Summary The annular cement sheath is one of the most-important well-barrier elements, both during production and after well abandonment. It is, however, well-known that repeated pressure and temperature variations in the wellbore during production and injection can have a detrimental effect on the integrity of the cement sheath. A unique laboratory setup with downscaled samples of rock, cement, and pipe has been designed to study cement-sheath-failure mechanisms during thermal cycling, such as debonding and crack formation. With this setup, it is possible to set the cement under pressure and subsequently expose the cement to temperature cycling under pressure as well. Cement integrity before and after thermal cycling is visualized in three-dimensional by X-ray computed tomography (CT), which enables quantification of and differentiation between debonding toward the casing, debonding toward the formation, and cracks formed inside the cement sheath itself. This paper describes in detail the development and functionality of this laboratory setup along with the experimental procedure. Several examples to demonstrate the applicability of the setup, such as tests with different types of casing surfaces and different rocks, are also shown.

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.…”

“…A limitation of this approach is that the estimated microannuli sizes are not really correct and give no accurate information about the actual microannuli geometry [9,50]. Figure 6 shows four examples of experimentally created cement-casing microannuli [9,41] and it is seen that they are not homogeneous and uniform. On the contrary, the microannuli geometries are heterogeneous and can be somewhat random.…”

“…Figure 6. Examples of X-ray computed tomography 3D visualizations of experimentally created microannuli at cement-casing interface: (a) due to cement shrinkage [9], (b) after full cement sheath cracking [9], (c) with mudfilm cured at ambient pressure [41], (d) with mudfilm cured at elevated pressure [41]. Resolution of CT images is approx.…”

“…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].…”

Specification for Permanent Plugging Materials

“…Bond strength failure, debonding, may eventuate from two different loading scenarios; shear load, and tensile load. These loads can be induced by thermal cycling, hydraulic forces, volume changes of material, tectonic stresses, or a combination of these [11][12][13]. The volume change could be due to shrinkage and may occur either during curing or after setting due to changes in downhole conditions.…”