Summary
A comprehensive semianalytical model has been built to investigate the effects of drilling and perforating damage and high-velocity flow on the performance of perforated horizontal wells. The model incorporates the additional pressure drop caused by formation damage and high-velocity flow into a semianalytical coupled wellbore/reservoir model. The reservoir model considers the details of flow in the vicinity of the wellbore, including 3Dconvergent flow into individual perforations, flow through the damaged zone around the wellbore and the crushed zone around the perforation tunnels, and non-Darcy flow in the near-wellbore region. The wellbore flow model includes the effect of frictional pressure drop. Both oil and gas wells are considered.
The expressions provided in this paper for additional pressure losses caused by perforating damage, drilling damage, and high-velocity flow can be used to optimize perforating parameters and decompose the total skin into its components (perforation pseudoskin, damage skin, and non-Darcy skin).
Introduction
The performance of oil and gas wells may be influenced by the simultaneous effect of mechanical skin, high-velocity (non-Darcy) skin, and completion pseudoskin factors. The skin factors caused by formation damage and perforating damage constitute the mechanical-skin factor. The extra pressure drop caused by high-velocity flow is known as the rate-dependent or non-Darcy flow factor. Compared to an ideal open hole, the wells with completions and other geometries such as perforations, slotted liner, or partial penetration may experience additional pressure loss or gain. The additional pressure change caused by wellcompletion and geometry is quantified in terms of pseudoskin factor. The combined effects of all the skin factors lead to a total skin factor that maybe estimated from pressure-transient data. The total skin factor, however, is not simply the sum of the individual skin components, and the computation of the individual skin components is not straightforward (the interaction between the individual components of total skin is nonlinear).
Many studies have concentrated on the effects of formation damage and high-velocity (non-Darcy) flow on well performance. For perforated vertical wells, McLeod's analytical model has been a widely accepted approximation to account for the additional pressure drop caused by formation damage and high-velocity flow. Karakas and Tariq presented a semianalytical model to predict the pseudoskin and productivity of perforated vertical wells with formation damage. The models suggested by McLeod and Tariq, however, may not work for selectively completed wells in which the flux distribution may be nonuniform. An example of this case is selectively perforated horizontal wells.
Tang et al. presented models for horizontal wells completed with slottedliners or perforations. The additional pressure drop in the vicinity of the wellbore because of formation damage, perforating, flow convergence, and high-velocity flow was included in their models in the form of a total-skinterm. The existing horizontal-well models are not capable of explicitly relating the skin factor to the physical parameters controlling the additional pressure drop around the wellbore. In addition, the interplay between the skin and flux distribution and its impact on the productivity of perforated horizontal wells have not been discussed, especially for selectively perforated horizontal wells. Non-Darcy flow effect in perforated horizontal wells is another topic that has not been addressed adequately in the literature.
In this study, we present a semianalytical model to predict the productivity of perforated horizontal wells under the influence of formation damage, perforating damage, and high-velocity flow. The nonlinear interaction between the individual skin components is accurately represented in the model. The model is applicable to both single-phase oil and gas wells (the pseudo pressure concept is used to extend the oil-flow model to the gas wells). Using the model, the combined effects of formation damage, the crushed zone around the perforation tunnels, and the high-velocity flow on the horizontal-well performance have been investigated in detail. The completion and damage parameters controlling the well productivity were identified through sensitivity studies.
Modified Horizontal Well Productivity Model for a Tight Gas Reservoir Subjected to Non-Uniform Damage and Turbulence