Prediction of Well Performance Instability in Thin Layered Reservoirs

Abstract

Abstract By using a full dynamic model for both the well and the near well reservoir, a large range of instabilities can be simulated. These instabilities include gas lift heading, severes slugging and compressibility effects. To simulate these types of fluctuations it is necessary to use an integrated dynamic model for the complete system. Using a constant productivity index to describe the inflow or using lift curves for the well are not sufficient to capture the dynamics. In this paper a well is described where reservoir-well interaction leads to unstable well behaviour. In order to investigate the cause for the unstable production a full dynamic model was developed for both the well and the near well reservoir. The simulation results are compared to field data of this well. The instabilities encountered at this well were explained by a combination of severe slugging in the well and the influence of the dynamics of the reservoir. The wellbore could act as a gas storage volume in which a pressure build up can take place. The well model used was a three phase dynamic models based of the drift flux formulation. For the reservoir basic single phase oil model was used to simulate a radial inflow. In the reservoir the compressibility of the oil is taken into account. Introduction Most oil and gas wells become unstable during the production lifetime. These instabilities are generally undesirable as the large fluctuating oil, water and gas rate at the wellhead will result in a fluctuating load on the surface equipment. This will in practice limit the average production and the ultimate recovery. Therefore, it is preferable to stabilize the production rates. However, in practice the instabilities can arise from a multitude of causes. Well-known sources are the occurrence of gas lift heading and slug flow. Lesser known instabilities arise from the interaction between the well and the reservoir. This can occur when a gas or water cone breaks through. Besides the dynamics of oil inflow due to the compressibility of oil can cause unstable well performance. In this paper, some of the sources are discussed. This will be done based on field data of an unstable well. The objective of the study was to investigate different sources of instability and to evaluate the conceqences for the performance of the well. The goal of the project is to increase the ultimate recovery of a well by controlling the inflow and thereby increasing the operation range. In this paper we focus on the stability analysis of the well-reservoir system and less on the controllability. For this analysis, a dynamic well simulation model is used, which can be coupled to a dedicated reservoir model. The reservoir models can range from a simple volume, to a complex gas-oil-water model. In the results presented in this paper, the reservoir model is restricted to near well reservoir models. Hereafter a basic description of the well is given and the encountered instabilities are discussed. Then the model is described in more detail. The steady state simulation results are used for a first analysis of the model. Then the different instability sources are investigated. The pressure levels and frequency obtained from simulating the various types of instabilities are compared to measured data. Well and reservoir description The well under consideration is a well with a total length of 3150m (see Figure 1). The horizontal section of the well is approximately 550m long. The perforated section is 600m long. This section is an open bore hole protected only with a sandscreen. The well produces from a homogenous sandstone reservoir (50 mDarcy). The production zone has a thin oil layer with a current approximate thickness of 20m.With the large height difference in true vertical depth of the horizontal section this means that some sections of the perforations are in or near the water layer. Originally there was no gas cap, but as the reservoir pressure decreased, small gas caps have been detected close to the well bore. The well is equipped with a gas lift injection point at an axial position along the tube of x=2250m. This gaslift is operated with a choked injection valve. The injected mass flow is therefore constant. One downhole pressure gauge is available at x=2270m.