Redefining Limits: Cement's ECD and Fracture Gradient in Drilling Operations in Highly Depleted Tight Gas Sandstone Reservoir, North of Oman
Authors: Aamri M. , Zadjali R. , Bisdom K. , Hattali A. , Lai S. , Sarfare M. , Hajri S. , Shaibani H. , AL Humaid M. , Ghallabi Y. , Yarabi M. 
Published: January 2025

In tight sandstone Field X, increased reservoir depletion has led to situations where Equivalent Circulating Density (ECD) during drilling and cementing exceeds the high case predicted fracture gradient (FG). The current FG range has been updated considering the depletion impact in producing reservoirs where low, expected, and high FG cases developed. However, this has resulted in cases where the drilling mud resulted in ECD &gt; FG_low or FG_Exp, as well as instances where the cementing resulted in ECD &gt; FG_Exp or FG_High. Due to the increased depletion in producing reservoirs, the current a ctual cementing ECDs exceed high-case FG in some of the highly depleted zones and, indeed, the margin will increase as reservoir pressure gets more depleted with time. To ensure safe and efficient well delivery, particularly with the planned drilling of approximately hundreds of wells over the next five years, it is essential to develop a refined FG prediction model for drilling and cementing those accounts for the impact of depletion.

The data used for this work includes information related to drilling operations, cementing, and fracturing parameters, which form the basis of this study. Fracture initiation pressure (FIP), fracture breakdown pressure (FBP), and fracture closure pressure (FCP) were determined based on data from fracturing operations using low-viscosity fluid (approximately 1 cp), which is suitable for constraining the drilling fracture gradient (FG) due to the fluid's properties. However, fracture propagation pressure from the main frac data using medium-viscosity fluid (up to 30 cp) was used as the basis for cement FG. Over 50 frac stages were utilized for this task. Based on the field's drilling and cementing history, mud weight (MW) ranged from 11 to 12 kPa/mTVD, with a drilling ECD of approximately 12-14 kPa/m and a cement ECD of 16-17 kPa/m, potentially reaching 18.5 kPa/m in slim design scenarios.

The current FG model for depleted units, using a depletion coefficient of 0.5, showed that the standard deviation between FG_low and FG_High was less than 1 kPa/mTVD. When plotting closure pressure data from all stages, 73% of newly collected data fell outside the existing FG model. By adjusting the depletion coefficient by 20% and updating the model by lowering FG_low and raising FG_High resulted in over 90% of the data falling within the model. FG_High was anchored to highest observed drilling ECD, validated by the absence of losses during operations.

After updating the drilling FG model which incorporated new depletion data, all propagation pressures from the main frac operations, which involved medium viscosity fluid of xxx cP, were plotted. It was observed that these pressures also trended downward with increasing depletion but remained higher than FG_High and the current cementing ECD envelope. This indicates that the nature of cementing fluids, which are more viscous, may result in a higher fra cture gradient than that used for drilling. Consequently, the study suggests that cementing fracture gradients should be adjusted similarly to drilling FG, using the same depletion coefficient of ∼ xxx. In addition, the impact of lost circulation materials (LCM) during operations was also reviewed, showing no significant trends affecting the FG.

The integration of new data and field experiences into the FG model increases confidence in well planning and execution, supporting sustainable field development. By addressing the challenges posed by reservoir depletion and aligning operational practices with updated FG predictions, this study provides a robust foundation for future well delivery in the studied field.