Experimental Investigation of Penetration and Plugging Performance of Cement Blends Used for Squeeze Cementing in Sub 150 Micron Gaps

Abstract Squeeze cementing is one of the most common techniques used to remediate well integrity issues. Understanding the mechanisms controlling cement particle penetration and/or bridging as well as assessing the potential of cement blends in narrow gaps are crucial for the design of a successful squeeze cement job. The main objectives of this study are to determine the penetration potential of different cement blends; determine the critical gap width where bridging of cement particles starts and to compare the fracture conductivity of the sample before and after remediation. Four different cement systems were tested for their ability to penetrate and plug narrow fracture gaps, including class G (A1) (largest particles), Portland limestone blended cement (A2) (medium particles), microfine cement (A3) (smallest particles), and Semi-microfine Class C (A4). Cylindrical cement plugs were cured at 1000 psi and 50 °C for 7 days and cut into halves to create replicas of fractured cement samples. Metal shims of 150, 100, and 50 microns in thickness were used to control the gap size. Cement micro squeezes jobs were conducted using a conventional core flow set-up. The fractured cement samples were imaged using micro-CT scanning technique before and after remediation to determine both the fracture width and the slurry flow pathway. A significant reduction (varying between three to six order of magnitudes) of the fracture conductivity was observed because of squeeze cementing, even though the cement did not fully penetrate the fractured cement samples in some cases. Squeeze cement slurry progression in the fracture takes place in several modes, all controlled mainly by the narrowest gap width size/particle size ratio (GW/PS ratio). At high GW/PS ratio, the cement slurry was distributed uniformly. At moderated GW/PS ratio, bridging and partial plugging of the fractures were observed, leading to slurry flow in fingered pattern. At low GW/PS ratio only filtrate flow was observed. Comparison of the results from all 13 cases of squeeze cementing experiments conducted throughout this study suggest that there may be a critical gap width/particle size (D90) ratio, which controls the particle bridging versus flow (and the depth of cement slurry penetration into the fracture) and this number is somewhere between 2.2 and 2.4. When the minimum fracture gap width/particle size (D90) ratio is around 2.2 and lower, cement slurry starts to bridge, thus preventing further filling of the gap by the squeezed cement. When the minimum fracture gap width/particle size (D90) ratio is sufficiently higher than 2.2, cement bridging does not occur, and the cement slurry completely fills the fracture. When the minimum fracture gap width/particle size (D90) ratio is much less than 2, the cement slurry flow stops suddenly in the narrowest region without any fingering and filtration. Preliminary results from this study suggest that remedial cementing penetration strongly correlates with fracture gap size/cement particle size (D90) ratio. However, more data, like the ones presented in this study, are needed before we can suggest a more accurate value of critical gap width/particle size ratio, which controls the probability of plugging in any squeeze job.