Effects of Foamy Oil and Geomechanics on Cold Production
Authors: Liu Y. , Wan R.G. , Jian Z. 
Published: March 2010

Foamy oil solution gas drive mechanisms are complex and our knowledge and understanding is limited despite extensive studies in the literature. In order to advance our understanding of heavy oil solution gas drive mechanisms, long core depletion experiments were designed. These experiments were performed on sand-filled or glass bead-filled tubes that are x-ray transparent and have pressure transducers along their length. The novelty of the experiments is the length that they extend (over 18 m) and the duration of the experimental runs. The results of the longer experiments should be able to provide data that bridge the gap between the field scale and the shorter laboratory experiments that have been performed in the past. Thus, production, pressure transient and saturation data are presented in this 'extended' scale. In ddition, CT scanner images are expected to provide information about the evolution of gas.

Introduction

Cold production, or more elaborately, Cold Heavy Oil Production with Sand (CHOPS), has been tried in unconsolidated or weakly consolidated sands as a non-thermal stimulation process in which both sand and oil are produced together in order to enhance oil recovery. The oil production process is also typified by the formation of a so-called foam(1) as a result of gas exsolution and dispersion of tiny gas bubbles with limited growth in size. An intriguing observation is that the resulting foamy oil flow seems to greatly enhance oil production rates with high primary recovery factors despite the high oil viscosity. There have been many explanations put forward for interpreting such a phenomenon; namely sand production, retardation of reservoir pressure decline, enhancement of absolute permeability and high critical gas saturation. This paper looks into some of the above-mentioned issues by exploring numerically the inter-relationship between sand production, sand failure and foamy oil flow during the enhancement of oil production in a non-thermal process such as CHOPS.

The Model

There has been a series of papers published by the authors on the topic of sand production modelling(2–4) in conjunction with geomechanical issues and, recently, foamy-oil flow(5–7). These form the basis and frame of reference for the modelling effort reported in this paper. The avid reader seeking details of formulation and computer implementation is thus directed to the above-mentioned references. In view of providing some background for the subsequent discussions, the main features of the sand production, foamy oil flow and geomechanics models are summarized in the next subsections.

Formulation

Basically, we are faced with a porous medium which is multiphasic in character consisting of gas (bubbles), oil, fluidized solids and solid phases. The oil contains dissolved gas which is liberated as the pressure drops below bubble point through gas exsolution, and thereby enters into the gas phase. For continuum mechanics modelling purposes, all above-mentioned phases are homogenized through a mathematical artifice within the theory of mixtures(8) (see Figure 1).

As such, mass balance equations can be written for each phase, and thereafter supplemented with two constitutive equations to

FIGURE 1: Representative Element Volume: discontinuous phases and homogenization. Available in Full Paper