Effect of Accelerational Pressure Drop in a Horizontal Wellbore

Abstract

Abstract It is desirable that a horizontal well is properly designed with regard to the production performance, as well as the well completion. A rigorous analysis of flow hydrodynamics in a horizontal well was performed by including the accelerational pressure drop in the wellbore. This has been recognized as one of the unsolved, yet most important problems in the production engineering. Pertinent experimental data were acquired with a large scale test facility, which was suitable for acquiring data on the relationship between the pressure drop along the wellbore and the fluid influx from the reservoir. An initial mechanistic model was improved by including the accelerational pressure drop in the wellbore. As evident from the comparison of the experimental data with the model, the improved model described more rigorously the flow behavior in a horizontal well configuration. The model would be applied for field applications, by combining with the Inflow Performance Relationship (IPR) approach and the black oil model. Introduction Literature Review Horizontal wells have become attractive for the production of thin layer reservoirs, naturally fractured reservoirs, and also reservoirs with gas or water coning problems. Horizontal wells can improve the inflow performance of these reservoirs, and produce more oil with smaller pressure drawdowns as compared with conventional vertical wells, due to enhancement of the reservoir contact and negative skin factors. Both the flow behavior in a horizontal wellbore, and its interaction with the reservoir, have been recognized as one of the unsolved, yet most important problems in the production engineering. Neither the pressure drop-flow rate behavior in the horizontal section, with increasing flow rates along it, nor the relationship between the pressure drop in the horizontal section and the flow in the reservoir has yet been clarified. These are also the essential items of information in proper design of a horizontal well. Therefore, further study is required on this subject. Despite the increasing number of publication pertaining to drilling and reservoir aspects of horizontal wells, a detailed literature search showed that several studies have been conducted on the flow behavior in horizontal wells. Dikken presented a simple analytical method that links a single-phase turbulent liquid flow in a horizontal wellbore to an isothermal reservoir flow, and predicts the frictional pressure gradient along the wellbore. Brice tested the Dikken's model against actual field data and confirmed frictional pressure drop in horizontal wellbores. He concluded that reservoir simulators do not predict the pressure drop in the wellbore if they are not corrected for diameter due to perforations in the production liner to represent actual flow conditions. Ozkan et al. presented a semi-analytical model coupling wellbore and reservoir single-phase liquid flow, and incorporating the effect of laminar and turbulent flow patterns in the wellbore. Ihara et al. have conducted experimental and theoretical investigations on this subject using a large scale test facility which features a 54.9-mm I.D., 105-m long horizontal well test section, as well as a small scale test facility which features a 25.4-mm I.D., 7.9-m long horizontal section. These test facilities closely simulate the interaction between a horizontal well configuration and the reservoir, and enable to acquire data on pressure drop and liquid-holdup. Although good agreement was found between the data and a physical model proposed by them, the model showed a discrepancy from the data at downstream where fluid velocities were relatively high. This model was combined with a single-phase inflow performance relationship to study horizontal well productivity. Brekke studied the effect of completion methods on horizontal well productivity and reported that frictional pressure drop due to restricted flow through perforations reduced productivity of the wells. He used a horizontal well simulator, supported by flow experiments, to test the performance of stingers and showed increased well productivity due to reduced frictional pressure drop in the wellbore. A field test on the North Sea wells using this completion technique showed reduced frictional pressure drops in horizontal wellbores. Problem Description It has been argued in the literature on the reservoir engineering that the infinite conductivity wellbore assumption is adequate for describing flow in horizontal wells. Although this may be a good assumption in situations where the pressure drop along the horizontal section of the wellbore is negligible compared to that in the reservoir, it is also reasonable to expect noticeable pressure drops in long horizontal wells. In practice, a pressure drop from the upstream end of a horizontal wellbore to its downstream end is essential to maintain fluid flow within the wellbore. Thus, the pressure distribution along a horizontal wellbore cannot be ignored (Fig 1). This is especially true when single-phase turbulent liquid flow or two-phase flow, including a compressible gas phase, is encountered in the wellbore. P. 125^