Modeling the Effect of Asphaltene on the Development of the Marrat Field

Abstract

This paper describes the study of the effect of asphaltene precipitation and deposition on the development of the Marrat field using a compositional simulation model with asphaltene modeling facilities. The model enables the simulation of asphaltene precipitation, flocculation, and deposition including adsorption, plugging, and entrainment, and the resulting reduction in porosity and permeability and changes in oil viscosity and rock wettability. A workflow was established in the study to i) characterize the equation of state (EOS) by analyzing the fluid PVT and asphaltene data from the lab; ii) calibrate the asphaltene model input parameters using the core flood experimental data; and iii) incorporate the EOS and the asphaltene parameters into the full field simulation model. The model was used to analyze the effects of asphaltene on various development scenarios, including depletion and water injection. For each scenario, the following were calculated and analyzed: field performance including production of oil, gas, and water; asphaltene behavior including precipitation, flocculation, adsorption, plugging and entrainment; formation damage and the effect of rock wettablity changes. The results show that the formation could be severely damaged near the producers where asphaltene is more likely to precipitate because of lower pressures (below the asphaltene onset pressure) and longer time exposure to a larger amount of oil. Formation damage could be reduced by flushing away the deposited asphaltene using higher flow rates if plugging is not significant. The water injection scenario with higher injection rate and higher production rate with BHP limit above 6,000 psia results in less asphaltene adsorption, more entrainment, and therefore less deposition. This in turn causes less permeability damage and more oil production. Introduction The Kuwait Oil Company (KOC) encountered the problem of asphaltene precipitation in Jurassic production wells located in producing areas in West and South East Kuwait. In West Kuwait (WK), the Jurassic production is primarily from the Marrat field. Out of about 45 wells in WK-Marrat 50% have a history of asphaltene cleanouts. These wells contribute to around 7% of the total oil production from WK which can amount to as much as 50 Mbbl/d. Here the reservoir pressure (around 9,500 psi) is considerably above the asphaltene onset pressure (AOP) (estimated between 2,000 psi to 4,000 psi). Therefore, there is no likelihood of asphaltene deposition in the reservoir. However, during production as the pressure of the produced fluid inside the tubing goes below the AOP, asphaltenes start to precipitate from the crude. Asphaltenes gradually deposit in the tubing, reducing its diameter, and in the process cause production rates to drop; eventually the well completely ceases to flow. Once this has occurred, the tubing in the well must be cleaned out to restore the well to production. The Marrat reservoir lies in the South Eastern part of Kuwait, within the giant Burgan field complex. It is a carbonate reservoir, existing at an average depth of 11,000 to 11,500 ft, subsea. The reservoir fluid is generally light, ranging in quality between 36 and 40 API degrees. The original oil in place is estimated to be two billion stock tank barrels. The original reservoir pressure at discovery was determined to be 9,650 psia. A log with the average properties of the reservoir is shown in Figure 1. The fluid is known to be asphaltenic 1 in nature, as evidenced by the deposition of asphaltenes in the production strings and surface production facilities, requiring periodic cleanup and treatment operations. In South and East Kuwait, all the wells in the Marrat reservoir were closed from 1998 to avoid further drop in the reservoir pressure, which had already dropped close to the AOP (the present reservoir pressure is around 8,400 psi and AOP ranges from 5,500 psi to 6,700 psi). Further reduction in reservoir pressure below the AOP could cause asphaltene deposition in the reservoir itself and result in formation damage. The decision was made to produce the wells only after a comprehensive development plan for reservoir pressure maintenance with implemented water injection.

Study of Asphaltene Precipitation and Deposition Phenomenon during WAG Application

“…Paraffin wax can deposit and form solid crystals due, mostly, to lowering of temperature. There are many papers that have addressed asphaltene problems during primary recovery or CO 2 injection as secondary recovery stage [5][6][7][8][9][10][11][12][13][14][15][16][17][18]. The reasons for the asphaltenes deposition can be many factors including variations of temperature, pressure, pH, composition, flow regime, wall effect and electro kinetic phenomena [3,4].…”

Review of Asphaltene Deposition Modeling in Oil and Gas Production

“…A model was developed by Yi et al 37 to simulate precipitation, flocculation, aggregation, and deposition of asphaltenes at the Marrat field in Kuwait. The reservoir pressure of 9500 psi was initially above the AOP, which indicates a slim chance of asphaltene deposition.…”

“…It was suggested that maintaining the reservoir pressure above the AOP would help in avoiding asphaltene issues in the reservoir and wellbore region. Yi et al 37 suggested severe effects on production when the pressure in the reservoir is below the AOP as a result of permeability reduction and wettability changes. In addition, the filed trial operating at a pressure below AOP could cause severe damage that could not be reversed.…”

Wettability Alteration by Asphaltene Deposition: A Field Example

“…Asphaltene is characterized by three additional components in the simulator: precipitation, flocculation and deposition (Schlumberger 2012;Fadili et al 2009;Boek et al 2011).…”