New, Generalized Criteria for Determining the Level of Underbalance for Obtaining Clean Perforations

Abstract

ABSTRACT: New criteria for estimating underbalance requirements for obtaining clean perforations are presented. These criteria are based on an anlysis of flow velocities and pressure gradients in the near wellbore region at early times using the element method, existing field data and a physical model of perforation cleanup. The results indicate that the physical model of perforation cleanup. The results indicate that the required level of underbalance is one that will introduce non-Darcy effects in the vicinity of perforations at early times. Simple equations based on pseudosteady-state, radial and turbulent flow in the crushed zone have been presented to approximate the required level of underbalance for a given set of formation and fluid properties. INTRODUCTION: Underbalance perforating, or perforating with a pressure differential into the wellbore, has been shown to improve productivity of wells under a variety of conditions. Underbalance techniques are now widely applied especially for natural completions. The key parameter in underbalance perforating is the level of presure differential that is needed for creating clean, undamaged presure differential that is needed for creating clean, undamaged perforaions and for removal of the damage in the crushed zone surrounding perforaions and for removal of the damage in the crushed zone surrounding the perforation tunnel. This is the minimum underbalance required for maximizing productivity of a well. On the other side of the scale is the maximum underbalance above which mechanical failure of the formation, casing collapse, packer unseating, or other problems within the well equipment may occur. The chosen value of underbalance for a completion should be between these limits with the exact value being a function of the degree of consolidation of the formation and/or the strength rating of the tubulars used in the well. For consolidated sandstones, the upper limit of underbalance is usually determined by the mechanical strength rating of well tubulars. Cleanup of the damage surrounding a perforation under downhole conditions is a complex phenomenon. It is affected by the characteris of the rock, the properties of the fluid saturating the pore spaces, and the manner in which flow is initiated after perforating. In this report, the results of a systematic simulation study of the flow and pressure conditions around a perforation immediately after its creation in pressure conditions around a perforation immediately after its creation in an underbalance environment are presented. Based on a detailed analysis of these results, an analytical formulation of pseudosteadystate radial, turbulent flow in the crushed zone, and their comparison with existing field data (Bell, King et. al.), it has been concluded that the required level of underbalance for cleanup is one that will introduce non-Darcy effects in the vicinity of perforations at early times. With non-Darcy flow around the perforation, the drag forces on fines and debris are sufficient to overcome viscous and gravitational forces and result in cleanup of perforations. The proposed model provides a consistent, physical basis for evaluation of the needed underbalance for different combinations of rock and fluid properties. Finally, a simple method based on pseudosteady-state radial, turbulent flow in the crushed zone has been devised to estimate underbalance for a given set of formation and fluid properties. STATEMENT OF PROBLEM Conventional perforating techniques result in some degree of damage - either temporary or permanent - to the target formation and numerous investigators have observed perforating damage under lab and field conditions. The high-speed, high-pressure jet causes the target material to low plastically in a radial direction creating a zone of compacted and crushed sand surrounding the perforations. In addition to the compacted zone, perforations are often plugged with mud solids, charge debris, metallic slug from the perforating charge and formation material. This plugging may also extend to the formation-matrix. The presence of other forms of damage, such as mud filtrate invasion or solid presence of other forms of damage, such as mud filtrate invasion or solid invasions from perforation fluid, will further magnify the damage. P. 215