Optimized perforation tunnel geometry, density and orientation to control sand production

Abstract

Abstract Destabilization of completed sandstone reservoirs reduces production rates and degrades production equipment. A major cause of such destabilization is perforation tunnel failure, which causes sand production. Classic mechanically-based sand control techniques are effective, but potentially unnecessary for some reservoirs. Modeling the occurrence and severity of sand production should help identify the most economical pairing of sand control remediation methods with the desired production rate. This paper examines an advanced technique for modeling such failures, leading to improved drilling and completion practices. In this study, 3D poro-elastoplastic Finite Element Methods (FEMs) are employed to model perforation tunnel stability. Wellbore geometry, reservoir properties, draw-down rate and perforation properties such as tunnel size, spacing and orientation are addressed, with particular attention focused on the transient phenomena near perforations, including stress re-distribution and failure development for different perforations. The results show that stability of the wellbore and perforation tunnel strongly depends upon drilling and perforation directions and perforation shot density. The relationships of the bottomhole flowing pressure, drawdown, perforation orientation, rock strength and in-situ stresses are given to provide optimal perforation for mitigating sand production. Introduction Sand production is an important issue affecting well production and casing stability. The major cause of sand production is wellbore instability and perforation tunnel failure in poorly- and un-consolidated formations. Classical sand control techniques are primarily based on installing gravel packing, frac-and-pack, etc., which dramatically increases the completion cost and reduces production. It is desirable to predict perforation tunnel stability and sanding potential before one makes a decision on whether or not to perform these costly sand control procedures. Sand production usually takes place in unconsolidated porous formations. To be effective, sanding prediction models need to address anisotropic and heterogeneous rock properties while accommodating the transient properties of the stress and fluid pressure fields around the borehole and perforation tunnels during production. In this study, a poro-elastoplastic analytical solution and 3D FEM are employed to model these complex properties. This advanced method allows modeling the anisotropy and heterogeneity of rock properties, in-situ stress, fluid pressure fields around the borehole and perforations arising from the changes that occur during reservoir depletion. The theoretical solutions are subjected to sensitivity analysis and model validation by examining a gas field in the Northern Adriatic Sea. The validated model can then be applied to design, drilling and completion operations in sand-prone reservoirs. Sanding Prediction Methods Sanding prediction is generally based on the following three categories 1,2: sand arch stability, open hole wellbore stability and perforation tunnel stability. Sand Arch Stability Lab experiments show that a sand arch is formed when sand is produced. The arch serves to support a load by resolving the vertical stress into horizontal stress. When the arch fails, the sand production begins. Figure 1 represents a sand arch failure causing sand production 3. Assuming an idealized production cavity and full spherical symmetry of the stress field, the following sand arch stability criterion was derived 4,5: Equation (1) where UCS is the uniaxial/unconfined compressive strength of the formation, q is the flow rate of the cavity, k is the formation permeability, r1 is the cavity radius and µ is the fluid viscosity.