Abstract

Heavy oil production methods included the cyclic steam stimulation (CSS), steam flooding and steam assistant gravity drainage (SAGD). It is well known, when water invasion occurs in heavy oil reservoirs with edge/bottom water, for a long time, how much of the produced water comes from the steam injection process as oppose to edge/bottom water invasion? Reservoir engineers do not clearly understand the process, because after the steam condenses a part of the condensation comes to the surface and another part is stored within the reservoir itself. This is the problem that still gives reservoir engineers trouble. Through studying the production data of thermal recovery at the Liaohe oilfield, in China, for the first time, this paper reveals a new theory and calculation method that shows how to calculate water invasion from the bottom/edge water reservoir, therefore, a new concept of conversion rate was presented and a new parameter of steam-water ratio was defined, as k. new equations of calculating produced water invasion and water invasion of the remaining formation with bottom/edge water were derived according to the new parameter k. In addition, an estimation method of breakthrough time of bottom/edge is presented in this paper. The methods have been used to calculate and analyze the production performance of heavy oil with steam injection in Liaohe oilfield. The calculation results accord well with actual situation of heat-recovery reservoirs. Therefore, this method will be recommended to optimize production performance parameters of thermal recovery.

Introduction

Currently, there are huge reserves of heavy oil in the world; however the physical properties of crude oil are very poor. Therefore a thermal recovery must be implemented in order to meet heavy oil production due to its high viscosity. CSS has been applied to commercial production of heavy oil reservoirs since the 1960s. During the 20th century, annual oil production reached 20 million tons, and thermal recovery technologies were greatly improved. CSS has become the main development technology for heavy oil development in the world.

Performance parameters of CSS surface operation consist of two parts: reservoir parameters and operation parameters. In order to efficiently improve reservoir management, reservoir engineers must understand performance parameters. For many years, reservoir engineers have studied parameters, such as: oil-steam ratio [1–5], production-injection ratio, back water production rate, and water storage rate [6–8] of heavy oilfields using CSS. However, formation water production, cumulative volume of formation water invasion, and formation pressure drop rate are still problems looking for better solutions. This research focuses on how to calculate the parameters in theory. This paper also presents the relationship among these parameters, and provides effective analytic method for performance analysis of heavy oil reservoirs with the aim of improving heavy oil reservoir management.

Reservoir Description

In the Liaohe oil field, China. These heavy oil reservoirs have been developed by CSS since the 1990s. They are characterized by heavy crude oil with high viscosity and bottom/edge water. According to lithology and storage space, they are divided into three types, naturally fractured metamorphics reservoir, dual porosity carbonate reservoir, and porous clastic rock reservoir. Heavy-oil fields are typically characterized as either (1) single sand, (2) multi-layer sands, or (3) massive block sands. Massive block sand is a single oil layer that is more than 10m thick and has a net to gross of more than 90%.

These fields exhibit crude oils with a large range of in-situ oil viscosity. There are conventional heavy oils with oil viscosity less than 10,000 mPa·s, ultra heavy oils with the oil viscosity between 10,000 mPa·s and 50,000 mPa·s, and super heavy oils with oil viscosity more than 50,000 mPa·s. Additionally, there is a wide range of reservoir depth: shallow where the depth is less than 500 m, mid-deep where the depth is between 500 m and 1000 m, deep with the depth between 1000 m and 1600 m, and ultra-deep where the depth is more than 1600 m.

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