Wellbore Strengthening by Diammonium Phosphate Treatment

Wellbore instability in chalk formations poses a significant and costly challenge for the oil and gas industry. Chalk formations often lack sufficient strength to withstand the stress concentrations encountered during and after wellbore construction. If these stresses exceed the rock's strength - a common occurrence in chalk - failure can occur around the wellbore. As oil and gas exploration ventures into increasingly harsh environments, including those featuring fragile chalk characteristics, maintaining wellbore stability grows ever more crucial. This study explores the viability of using diammonium phosphate (DAP) as a possible remedy. Chalk plugs measuring 1.5 inches in diameter underwent non-destructive mechanical assessment via acoustic velocity measurements, allowing us to calculate their dynamic Young's modulus and Poisson's ratio. Permeability and porosity evaluations complemented this process. Based on assessments of ultrasonic and petrophysical attributes, 4 defect-free plugs were identified and split evenly between an untreated control group and one receiving the DAP treatment. For the latter, specimens were subjected to a pressurized (1000 psi), heated (75°C), and concentrated DAP solution (1M) environment over a period of 72 hours before undergoing further examination. Subsequent static testing evaluated changes in Young's modulus, Poisson's ratio, and compressive strength at varying confining pressures (800, and 1200 psi). Ultimately, confined compressive strength (CCS) values derived from both sets facilitated creation of Mohr-Coulomb failure envelopes illustrating differences between treated and pristine chalk samples. Initial screenings confirmed uniformity among test subjects due to negligible variations observed throughout porosity readings (23-26%), permeability ranges (30-40mD), Young's moduli, and Poisson ratios. Conversely, application of DAP led to substantial improvements in mechanical performance. Specifically, post-treatment increases included approximately 16-29% boosts in peak stress. Moreover, notable gains emerged when comparing Mohr-coulomb failure envelopes obtained pre-and-post-DAP exposure-treated samples demonstrated higher cohesion and unconfined compressive strength (UCS) relative to unaltered counterparts. These results translate to a clear upward shift in the Mohr-Coulomb failure envelope for treated samples. This promising outcome demonstrates DAP's potential to significantly strengthen chalk, as evidenced by improved elastic and failure parameters. This has significant implications for mitigating wellbore failure and enhancing wellbore stability, potentially leading to a breakthrough in the field.