Effects of Non-uniform Skin Distribution on Horizontal Well Inflow Performance

Abstract

Formation damage around a horizontal wellbore has been a long-standing problem. The damage zone is expected to be a cone-like shape, extending from the heel end to the toe. This situation leads to a difficulty in studying the inflow performance. In fact, previous studies assumed clean and homogeneous formations and ignored any skin factor in predicting pressure drops along a horizontal wellbore, critical rate, and water breakthrough time. The present study is focused on investigating influx and flowing pressure distribution along a horizontal wellbore in the presence of non-uniform formation damage. The relevance with the inflow performance is also discussed. Modified wellbore hydraulics correlations coupled with a reservoir inflow equation are used to accommodate this effort. Calculations are performed for different skin distribution, damage severity quality, and wellbore diameters. Results obtained show that non-uniform skin causes reduced influx rate toward the heel end. The undamaged well reveals an opposite trend as expected. For a given total production rate, the influx rate at some portions of the wellbore increases as the formation damage gets heavier, as well as for smaller wellbore diameter, resulting in greater pressure drops. Thus, the presence of non-uniform skin promotes an increase in pressure gradients within the wellbore. The changes in pressure losses are more sensitive to the level of damage severity in smaller wellbore diameter. Such a study has not been previously reported in the literature. Overall, this investigation improves our knowledge to better understand flow behavior in a horizontal wellbore. The paper presents information to be considered in predicting the inflow performance and also provides a guide for deciding or optimizing a horizontal well completion strategy. Introduction Invasion of mud filtrate into a productive zone during drilling the well has been a long-standing problem. The residing mud filtrate and also the mud particulates can cause damage to the zone by reducing the permeability around the wellbore. This in turn results in higher pressure losses in the vicinity of the wellbore and thus reduces productivity of the well. Any damage around wellbore is therefore never desired because formation damage is also costly. When one gets his well damaged then his company unfortunately has to spend a lot of money and time for damage removal in order to put the well back on its natural capacity. Drilling a horizontal well causes the productive zone being exposed to the drilling fluid for a quite long period. A portion of the wellbore, particularly nearby the heel end, has the longest exposure time and therefore has the greatest skin radius. In a sensitive formation, the damage can be so severe that the horizontal well is irreparable. In the case of conventional vertical wells, Hawkins1 presented a model and the related equation for estimating the skin factor that can be used for calculating flow efficiency of a vertical well. To model skin distribution along a horizontal wellbore, Frick and Economides2 have successfully made representatives of formation damage distribution for horizontal well in isotropic and anisotropic reservoirs and come up with an analytical expression for damage. Many studies have been conducted3–9 to evaluate the effects of formation damage on horizontal inflow performance. These studies, however, have mainly focused on the well productivity or flow efficiency. In a comparison with conventional vertical wells, a relatively high ratio of horizontal-to-vertical well productivity can result in serious production losses of the horizontal wells. Some other investigations10–16 have looked deeper at inflow mechanism along a horizontal wellbore, including aspects of wellbore hydraulics. The results in general show that wellbore pressure losses can be quite significant when production rate is considerably high and/or wellbore diameter is relatively small. Unfortunately, formation damage that frequently occurres in horizontal wells has been ignored in such investigations.

Study on Skin Factor and Productivity of Horizontal Well after Acidizing with Nonuniform Damage

“…For anisotropic medium, Permadi et al [2][3][4][5] pointed that the shape of damage region was circular near the well and elliptical far from the well and presented a new analytical model for damage skin factor and the resulting reservoir inflow for a horizontal well, which can be more accurate than previous model for skin factor and productivity of HW.…”

The Impact of Nonuniform Formation Damage on Long-Term Well Performance

“…15 However, a multisegment horizontal well model is required in conjunction with the use of conventional method for formulating the non-uniform skin profile. [10][11][12][13][14][16][17][18] If the segment j along a horizontal well is under the influence of a local skin factor defined as After the incorporation of the scaled segment skin factor into the multisegment horizontal well model, the set of equations are simultaneously solved for flux distribution and pressure losses.…”

“…[8][9][10][11][12][13][14] Pressure transient characteristics of horizontal wells influenced by formation damage have been studied by many researchers since the mid 1980s. [15][16][17][18][19][20] Goode and Thambynayagam 15 were among the first to incorporate the effect of uniform skin factor into the transient pressure solutions for horizontal wells.…”