An experimental and theoretical investigation of the flow behavior in horizontal wells and its interaction with the reservoir was performed. This has been recognized as one of the unsolved, yet most important problems in the production area.
A new test facility was designed and constructed, suitable for acquiring data on the relationship between the pressure drop along the well and the fluid influx from the reservoir. The test facility features a 1-in. I.D., 26-ft long horizontal well test section flowing into a 9.2-ft high vertical section. Pertinent preliminary data were acquired.
Initial models are proposed which describe the flow behavior in a horizontal well configuration for both the experimental work and field operations. The models use the EPR (Inflow Performance Relationship) approach, the black oil model and mechanistic models for wellbore hydraulics.
A sensitivity analysis was performed on key parameters using the model for field operations. Good agreement was found between the model for the experimental work and the preliminary experimental data.
A new EPR apart from the extension of the Darcy's law must be investigated extensively to help in the proper design of horizontal wells.
Horizontal wells have become attractive for the production of thin layer reservoirs, naturally fractured reservoirs, reservoirs with gas or water coning problems, and also in offshore environments where various wells are drilled from a central platform. Horizontal wells can improve the inflow performances 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.
However, both the flow behavior in the horizontal wellbore, and its interaction with the reservoir, have been recognized as one of the unsolved, yet most important problems, in the production areal. Neither the pressure drop-flow rate behavior in the horizontal section, with increasing flow rates along the well, 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 the proper design of a horizontal well. Therefore, further study is required on this subject.
Despite the increasing number of publications pertaining to drilling and reservoir aspects of horizontal wells, a detailed literature search showed that only a few studies have been conducted on the flow behavior in horizontal wells. In 1989, Dikken presented a simple analytical method that links a single-phase turbulent flow in a horizontal wellbore to an isotherm reservoir flow and predicts the frictional pressure gradient along the horizontal Wellbore. He concluded that the reduced drawdown due to turbulent flow along the wellbore may result in the total production rate reaching a certain critical value as a function of wellbore length. Islam and Chakma presented a physical model based on experimental results that may describe multi-phase flow through perforations in a horizontal well. Heavy or mineral oil water, and air were used in their experiments. However, the interaction between the flow behavior in the horizontal well and the reservoir was not taken into account well in their experiments. Ihara et al. have recently conducted an experimental and theoretical investigation on this subject using a large scale test facility which features a 54.9-mm I.D., 100-m long horizontal well test section. This test facility closely simulates the interaction between the reservoir and horizontal well piping system and enables to acquire data on pressure drop and liquid holdup pertaining to the interaction. No other studies have been found on this subject.
The assumption of a constant pressure along a horizontal wellbore which is often used for well test analysis and reservoir simulation studies for horizontal wells, must be examined carefully. Neither a uniform-flux nor a uniform-pressure boundary condition in the wellbore is realistic for horizontal wells. In practice, some pressure drop from the upstream end of a horizontal wellbore to the downstream end is essential to maintain fluid flow within the wellbore. Thus, the pressure distribution along a horizontal wellbore cannot be ignored (Fig. 1) and has a very important effect on the production performance. This is especially true when two-phase flow, including a compressible gas phase, is encountered in the wellbore. For the production of single-phase liquid, pressure drop along the wellbore may be neglected, except for a high viscosity liquid, high reservoir permeability of a few darcies, or high production rates in excess of a few thousand reservoir barrels per day.