New Methods for Improved Oil Recovery of Thin Oil Rims
Authors: Zakirov S.N. , Zakirov I.S. 
Published: December 2007

An efficient way to influence critical gas free oil rates has been discovered. This idea was used to design a novel technology of thin oil rims development. Efficiency of this technology was studied for four types of oil rims differed in thickness of oil column and reservoir properties. It is shown that developed technology enhances oil recovery. An additional version of novel technology is presented having positive impact on cumulative oil production over life time of the field.

Introduction

Low oil recovery factors of thin oil rims are usually connected with gas/water breakthrough. Muskat and Vyckoff were the first to suggest a theory of steady state coning problem for oil formations with bottom water. After that, some theoretical and experimental investigations were carried out in order to extend this theory for critical gas/water free oil rates and water free gas rates estimation. Unfortunately, the critical gas free oil rates in case of thin oil rims drained by vertical wells turned out to be either low or unprofitable.

Situation has significantly changed alter world-wide application of horizontal wells for oil rims development. Theoretical basis for gas/water free oil rates of horizontal wells was created. A high efficiency of critical gas free oil rate regime was confirmed by simulation and during long-term test production of two horizontal wells on the unique Troll field.

At the same time some studies were devoted to suppress gas/water coning process. Under certain circumstances a barrier flooding seems to be rather efficient. It is supposed to create immovable barrier along gas-oil contact from gel, foam etc. Both screen creation and barrier flooding were recommended in paper. There are some approaches consisting in dynamic pressure barrier creation along GOC or WOC. It is worth to be mentioned the suggestion based on injection of active admixture below WOC that alter interaction with oil forms the impermeable combination. Performed lab studies and theoretical investigations unexpectedly led to the new approach of oil rims development that consists in water injection above GOC and gas injection below WOC.

In spite of variety and extensiveness of the methods of oil rim development the problem of enhanced oil recovery of thin oil rims still exists and it is one of the most challenging problem at the moment. First of all it is connected with ultimate oil recovery factor (ORF) does not usually exceed the level of 15-20%.

The essence of novel approach

Let us consider reservoir model in fig. 1. Two horizontal wells are located on the left and right edges of the reservoir. As far as the fragment of reservoir pattern is considered these two wells contribute only as half well due to reservoir symmetry. Let the left horizontal well be operating under critical gas free oil rate. The right horizontal well for water injection is used. However, such technology is not new for vertical wells implementation. It was applied, for example, to Lantorskoe and other West Siberian fields. That technology has not ensured high values of oil recovery factors almost for all 9-point patterns corresponding to different reservoir conditions. It is usual for conventional technology that injected water displaces oil rim toward gas cap that breaks down oil rim reserves. Producing wells operate under super critical rate mode that results in huge gas-oil ratios and high water cut.

Unlike mentioned above technology, the pressure maintenance should be designed under consistent flow rates with the left producing well operated at critical gas free rates. It is supposed that due to influence over oil rim it will be in tension state. This state should lead to certain counteraction of gas coning process. If it is true then the current critical gas free oil rates of the left producing well should decrease more slowly and will be higher comparing with depletion drive. P. 261