Downhole Electrical Heating for Enhanced Heavy-Oil Recovery
Authors: Carpenter Chris 
Published: February 2015

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper IPTC 16858, ’Downhole Electrical Heating for Heavy-Oil Enhanced Recovery: A Successful Application Offshore Congo,’ by F. Bottazzi, C. Repetto, E. Tita, and G. Maugeri, Eni, prepared for the 2013 International Petroleum Technology Conference, Beijing, 26-28 March. The paper has not been peer reviewed. Copyright 2013 International Petroleum Technology Conference. Reproduced by permission.

Several enhanced-oil-recovery (EOR) techniques (thermal methods in particular) are currently available to achieve higher recovery factors in heavy-oil fields. However, some technologies, such as downhole electrical heating (DHEH), have been improved recently, allowing enhancement of production and therefore improvement of the recovery factor with relatively low investment cost in comparison with the high costs involved in the implementation of thermal technologies. To increase the production of a heavy-oil reservoir offshore Congo, a study of DHEH applications has been carried out with encouraging results.

Introduction

The Zatchi Marine field, operated by Eni Congo in partnership with Total Congo, is located approximately 25 km from the Congolese coast (Fig. 1). It is a multilayered sand reservoir with five oil-bearing levels (A through E). In particular, Level B is a 30-m-thick sand characterized by a large accumulation of heavy and highly viscous oil with the presence of both a bottom and a gas cap and very low reservoir pressure. Water depth ranges from 55 to 57 m, and there is an areal extension of approximately 34 km2.

The development of the Zatchi B reservoir has always been an extreme challenge. Since the development of the field, six wells have been drilled and only three have been put into production, with sometimes unsatisfying results because of the severe problems of gas coning and cresting. An important improvement was achieved in 2008 with the drilling and completion of Well ZAM-408 ML, the first successful multilateral well ever drilled in West Africa and the first Technology Advancement of Multilaterals Level 6 technology applied in the Congo basin.

Following this important achievement and the growing interest in the various EOR techniques and development scenarios, a dedicated study was conducted by Eni to evaluate and assess the most suitable integrated technology applicable to the Zatchi B reservoir.

Reservoir simulations showed that among the different EOR techniques, thermal methods could enhance the recovery factor from this reservoir significantly. However, because the study is considering an offshore field (producing from a platform with very limited space and available power supply), and taking into account the heterogeneous reservoir environment, thermal methods were deemed to be too costly, with a high risk and a long-term implementation. Therefore, another technique was taken into consideration: cold production assisted by a DHEH system, which represents the edge between cold and hot production. DHEH allows production enhancement and thus improvement of recovery factor, with significantly lower investment costs when compared with those typically associated with the implementation of thermal technologies such as steam injection. This technology uses a three-phase system designed to provide a defined wattage according to the different applications and type of cables. The heater section is set downhole and is connected to the surface with a power cable. It generates heat to the near-wellbore region, decreasing viscosity and friction and, consequently, increasing oil mobility.