Sand Production Prediction in High Rate, Perforated and Open-hole Gas Wells

Abstract

This paper presents an analytical model for the prediction of the on-set of sand production or critical drawdown pressure (CDP) in high rate gas wells. The model describes the perforation and open-hole cavity stability incorporating both rock and fluid mechanics fundamentals. The pore pressure gradient is calculated using the non-Darcy gas flow equation and coupled with the stress-state for a perfectly Mohr-Coulomb material. Sand production is assumed to initiate when the drawdown pressure condition (i.e. atCDP) induces tensile stresses across the cavity face. Both spherical and cylindrical models are presented. The spherical model is suitable for cased and perforated applications while the cylindrical model is used for a horizontal open-hole completion. For input, the model requires cohesive strength and an internal friction angle that characterizes a Mohr-Coulomb material; preferably predicted using a log-based mechanical properties algorithm in order to generate a foot-by-foot profile of the maximum sand free drawdown for gas wells. The example GOM well illustrates a continuous profile of critical drawdown with depth, providing quick identification of potential sand producing zones. This allows a gravel pack decision to be made in the period between logging and completion. It also facilitates the design of selective perforation programs. The model demonstrates that non-Darcy flow has a considerable effect on the sanding tendency of weak but competent rock, and completion decisions in high gas-rate wells that neglect the influence of non-Darcy flow could be overly optimistic. It also shows that theCDP of a horizontal well with slotted liner is less than that of the corresponding cased and perforated completion. Introduction High-rate gas well completions are common practice in offshore developments and among some of the most prolific gas fields in the world. These fields typically have reservoirs that are highly porous and permeable but weakly consolidated or cemented, and sand production is a major concern. Because of the high gas velocity in the tubing, any sand production associated with this high velocity can be extremely detrimental to the integrity of surface and downhole equipment and pose extreme safety hazards. Prediction of a maximum sand free production rate is therefore critical, not only from a safety point of view but also economically. The unnecessary application of sand controlled techniques, as a precaution against anticipated sand production, can cause an increase in completion costs and a possible reduction in well productivity. However, if operating conditions dictate the need for sand exclusion, such techniques can make a well, which otherwise could have been abandoned or not developed, extremely profitable. The ability to accurately predict CDP is, therefore, critical to optimizing the completion strategy. Two mechanisms responsible for sand production are compressive and tensile failures. Compressive failure refers to tangential stresses near the cavity wall exceeding the compressive strength of the formation. Both stress concentration and fluid withdrawal can trigger this condition. Tensile failure refers to tensile stress triggered exclusively by drawdown pressure exceeding the tensile failure criterion. Veeken et al. 's1 review noted that laboratory and production experiments support the existence of both types of failure mechanisms; with tensile failure predominating in unconsolidated sands and compressive failure in consolidated sandstone. The consensus is that near borehole stresses cause desegregation of the formation while the fluid drag forces provide the medium to remove the failed materials.

The effect of Non-Darcy Flow on Induced Stresses Around a Wellbore in an anisotropic in-situ stress field

“…They have concluded that non-Darcy flow will significantly increases the instability of perforation tunnels [24]. Ong et al (2000) have combined equilibrium equations with the MohrCoulomb failure criterion and non-Darcy fluid flow to study the onset of sanding in spherical or cylindrical perforation of high rate gas wells. They have inferred from the result of specific reservoir that the non-Darcy flow reduces the critical drawdown pressure by half as compared to the Darcy case [6].…”

“…Ong et al (2000) have combined equilibrium equations with the MohrCoulomb failure criterion and non-Darcy fluid flow to study the onset of sanding in spherical or cylindrical perforation of high rate gas wells. They have inferred from the result of specific reservoir that the non-Darcy flow reduces the critical drawdown pressure by half as compared to the Darcy case [6]. They also have proposed the method of predicting the onset of formation solid production [25].…”

“…The impact of the flow of hydrocarbon when enters into the wellbore on the induced stresses around the wellbore have been reported by researchers [6] [7][8] [9]. For example the tangential stresses were increased due to stress concentration and fluid withdrawal that leads to compressive failure around a cavity of a well [6] or when the pore pressure gradient is very high, the effective radial stress may be tensile [8].…”

“…For example the tangential stresses were increased due to stress concentration and fluid withdrawal that leads to compressive failure around a cavity of a well [6] or when the pore pressure gradient is very high, the effective radial stress may be tensile [8].…”

“…laminar flow assumption near wellbore [3] [7][14] [17]. There are only few studies which assumed non-Darcy flow to developed sand production model for high rate gas wells [6] or derived modified equations for non-Darcy effect on isotropic stress field [9].…”