Oil Production and Water-Cut in Naturally-Fractured Vuggy Reservoirs

Abstract

Naturally fractured oil reservoirs with vugs of size over 5 meters in diameter are few in the world. We developed a method to correlate the decrease in oil production and the increase in water cut. Production data from Tah oil reservoir (China) with big size vugs and high-density fractures were used to test the approach. It was found that the oil production decline and the water cut increase were coupled closely in many wells from this naturally fractured vuggy oil reservoir. The oil production was characteristic of rapid decline and the water cut was characteristic of sharp increase right after water breakthrough in many oil wells. An idea of shutting off water before water enters the production zone was proposed to reduce the decline rate of oil production and the increase in water cut. This approach of water shut off has many advantages compared to the routine measures which are usually taken after water cut is already great. Introduction A large amount of the world's hydrocarbon reserves are in carbonate reservoirs with natural fractures and vugs. These carbonate reservoirs are more challenging for estimating the petrophysical properties and understanding the fluid flow mechanisms as well as the production performance, compared to most sandstone reservoirs (Hurley, et al., 1998). This may be because the rock in carbonate reservoirs contains not only matrix and fractures but also different size of vugs. Although there have been many reports on decline curve analysis (Arps, 1945; Fetkovich, 1971 and 1980; Agbi and Ng, 1987;Camacho and Raghavan, 1989; Palacio and Blasingame, 1993; Masoner, 1998; Agarwal, et al., 1999; Marhaendrajana and Blasingame, 2001), papers on naturally fractured reservoirs with big vugs of diameter ranging from about one centimeter to over 5 meters have been few. Li and Horne (2005) proposed a mechanistic decline model based on previous theoretical and experimental studies (Li and Horne, 2001, 2004). This model has been used in different cases by many researchers (Høgnesen et al., 2004; Arihara, 2004, Reyes et al., 2004; Cunningham and Lecompte, 2005; Lawal, et al., 2008). However the Li-Horne model has not been applied to an oil reservoir with big vugs of diameter over 5 meters. It is important and critical to accurately predict the water cut for many aspects of reservoir engineering. The traditional aspects include reserves estimation, reservoir surveillance and management; the more modern aspect includes the need to comply with increasingly stringent regulatory requirements for the disposal of produced water. In many cases, decline of oil production is associated with increase in water cut. There are several models in the literature dedicated to fitting water cut trends during oil decline. As pointed out by Lawal, et al. (2008), these correlations can be divided into three main classes:using fractional flow theory, in which relative permeability functions are approximated, to establish water cut (or water-oil ratio) variation with oil recovery (Ershaghi and Omoregie, 1978; Lo et al., 1990; Sitorus et al., 2006);using the Arps model and its modifications, for example, semi-log water cut versus oil recovery, andobserved trends, for example, linear water cut versus oil recovery. While these methods have been applied extensively, none has been found to be sufficiently robust. In this study, we proposed a method to correlate the decrease in oil production rate and the increase in water cut in one graph. Production data from Tah oil reservoir (China) with large vugs and high-density fractures were used to test the approach.

Studies on the Factors Affecting Watercut of a Single Well in a Complicated Fault-block Reservoir by Applying the Numerical Simulation Method

“…Ivanova (2008) found regularities and confirmation watercut by means of analytical and numerical calculations. Li et al (2008) developed a method to correlate the decrease in oil production and the increase in water cut. It was found that the oil production decline and the water cut increase were coupled closely in many wells from this naturally fractured vuggy oil reservoir.…”

Seleção da estratégia de produção de um reservatório fraturado sob incerteza

“…Foi citado que a recuperação de óleo deste campo teve um incremento de 33% com a implantação da injeção de água. Li et al (2008) cita que o reservatório Tah (reservatório fraturado e vugular localizado na China) tem rápido declínio da pressão e aumento acentuado do corte de água assim que a irrupção de água atinge os poços.…”