Particle Size Distribution Acid Soluble Cement

Abstract

Historically, horizontal wells in the Bakken/Three Forks formations in the Williston Basin have been completed using open hole completions to maximize wellbore access. As the play was developed, operators moved to multistage completions using open hole packers. Lately, operators have been seeing various advantages of cemented laterals, including the opportunity for additional stimulation stages and the potential improvement of production. This has given rise to a field wide move towards cemented laterals. One challenge associated with cemented laterals can be increased near wellbore friction pressure during stimulation. This increased near wellbore friction pressure is related to the tortuosity as the stimulation treatment travels around the wellbore from the perforation tunnel to the primary fracture plane.Due to the increased near wellbore pressures during hydraulic stimulation in cemented laterals, acid treatments are frequently performed before hydraulic stimulation. Portland cement does not exhibit significant solubility in acid. For this reason, calcium carbonate is sometimes added to the cement to increase the solubility in acid. This paper aims to quantify the acid solubility with respect to time of some typical horizontal cementing solutions, and to determine the optimal cementing solution for minimizing near wellbore friction in cemented laterals. In an idealized situation, the near wellbore pressure could be decreased to nearly zero if all of the cement in the perforated interval was acidized away, while still leaving good zonal isolation of the cement sheath along the wellbore where there are no perforations.Laboratory testing shows that conventional cement designs exhibit a very small weight reduction when soaked in acid, and that reduction requires a long period of time to be achieved. Acid-soluble cement designs which use small concentrations of calcium carbonate exhibit slightly increased weight reduction after soaking in acid, but still requires a long soaking time for the dissolution to occur. To achieve maximum solubility in acid, the calcium carbonate concentration needs to be increased dramatically.When adding large quantities of non-cement materials, like calcium carbonate, to a cement slurry, the slurry properties and set-cement properties can be compromised. To achieve the required slurry and set-cement properties while achieving maximum calcium carbonate concentration and therefore acid solubility, a multi-modal particle size distribution cement design was used. This particle size distribution design allowed high weight reduction of after a short soaking time in acid, while still achieving a cement design that met all of the industry accepted best practices for cementing laterals.