SPE Members

Abstract

The cyclic injection of steam to stimulate or produce wells in heavy oil reservoirs is extensively employed by the oil industry. The performance of such wells may be predicted from empirical correlations, simple analytical predicted from empirical correlations, simple analytical models or thermal reservoir simulators. Empirical correlations can be extremely useful for correlating data within a field and for predicting performance of new wells in that and similar fields. However, use of such correlations for situations much different from the ones that led to their development can be subject to large errors. On the other hand one can use a compositional thermal model to predict the performance of cyclic steam operations. While predict the performance of cyclic steam operations. While thermal models are based on the fundamental laws of conservations, the fluid flow is related to pressure gradient through the empirical concept of relative permeability. In addition, the inherent sophistication of a thermal model makes it sensitive to rock properties, fluid properties and geological features. Much of this information is often unknown and must be estimated from limited data and experience in similar situations. Furthermore, because of the complexity of the recovery process - which involves reversal of flow directions - the equations of a thermal simulator are difficult and expensive to solve. All of this means that the use of a sophisticated thermal model may not be appropriate for routine design of cyclic steam operations. Instead one must consider the development and use of simple analytical models which give due consideration to important mechanisms involved in the process.

In this study existing analytical models for cyclic steam are reviewed and a new model proposed. This model incorporates many of the features of existing models with some important refinements. The flow rate of oil in the model is influenced by oil viscosity, effective permeability of the heated zone, porosity, mobile oil permeability of the heated zone, porosity, mobile oil saturation and thermal diffusivity of the reservoir. The change in reservoir temperature with time is also modelled, and it results in the expected decline in oil production rate during the cycle. The model accounts for the heat remaining in the reservoir from previous cycles. The model equations are kept as simple as possible and, wherever appropriate, correlations are incorporated to minimize data requirements.

The results of the model compare well with available field data and another analytical model. The model results are also compared with a thermal simulator.

Introduction

Cyclic injection of steam in heavy oil reservoirs is an important stimulation and oil recovery process. The steam is usually injected at a fixed rate and known wellhead quality. After some heat loss in the wellbore the steam enters the reservoir. The bottomhole quality and pressure may be predicted from a wellbore model of the pressure may be predicted from a wellbore model of the type discussed by Fontanilla and Aziz. After injection for a specified period of time the well is shut-in and the steam is allowed to "soak" into the reservoir for another specified period. To complete the cycle the well is produced until the oil production rate reaches some produced until the oil production rate reaches some minimum economic rate. This cyclic process is repeated until the recovery per cycle drops below some economic limit. Given the bottomhole conditions during the production cycle, the wellhead conditions may be predicted. production cycle, the wellhead conditions may be predicted. There are many options available to the engineer for the prediction of reservoir response to cyclic steam. These include multicomponent, multiphase thermal simulators, analytical models and simple correlations. While, in principle, the reservoir simulator should yield the most principle, the reservoir simulator should yield the most accurate answer, this may not necessarily be always true. The main reason for this is that the reservoir simulator is sensitive to data that are often not known or unreliable. It is natural then to try to develop simple analytical models or correlations that account for the important mechanisms involved in this process. This indeed has been the case and several models and correlations of varying degree of complexity are available in the literature.

Steam injected in a heavy oil reservoir tends to rise to the top, and the oil heated by the steam is produced by gravity drainage and pressure drawdown. The analytical models available in the literature for predicting the performance of cyclic steam operations are of two types:

  1. performance of cyclic steam operations are of two types:

    1. frontal displacement, and

    2. steam overlay.

    Figures (1) and (2) show the assumed distribution of fluids in these models.

The objective of this paper is to present a simple model that predicts the oil and water rates in heavy oil pressure depleted reservoirs. Such a model is presented pressure depleted reservoirs. Such a model is presented and results are compared with limited field data and some results from a thermal simulator. These results provide encouragement for further work in this area.

This content is only available via PDF.
You can access this article if you purchase or spend a download.