Frac/Pack Modeling for High-Permeability Viscous Oil Reservoirs of the Duri Field, Indonesia

Abstract

Summary This paper documents frac/pack completions conducted in the Duri field for the shallow, highly permeable, and unconsolidated formations containing heavy viscous crude. A treatment design model with a pressure-dependent leakoff option is presented to investigate effects of fracturing fluids, reservoir mobility, and Young's modulus on the fluid leakoff, net pressure, and fracture propagation. Field treatment data are analyzed with the provided model, and several factors that have significant influence on treatment results are discussed. Introduction Duri field is located in the eastern coastal plain of central Sumatra, approximately 70 miles northwest of the city of Pekanbaru. Oil is produced from a structurally trapped Miocene sandstone at depths between 200 and 900 ft. Duri field encompasses 30,000 acres and holds an estimated 5.4 billion bbl of original oil in place. It is the second largest oil field in Indonesia in terms of original oil in place. Rindu and Pertama/Kedua sands are two major producing intervals in the Duri reservoir where the combined pay thickness averages 250 ft and is located at depths between 250 and 700 ft. The formation has initial oil saturation of about 55%, average porosity of 34%, and air permeability in the pay formation ranging from 200 to 5,000 md. Currently, reservoir pore pressure is between 85 and 180 psi. The initial reservoir temperature is 92°F. The crude gravity is about 21°API (400 to 900 cp at the initial reservoir conditions). More information on typical reservoir properties is given in Table 1. In Duri, frac/pack became an attractive completion option owing to potential problems of formation damage and sand production. The first frac/pack pilot for the Duri field was conducted in 1996 for the Rindu Vertical Expansion Project.1 Some 500 wells of the Duri field were treated with cased-hole frac/pack techniques in 1999. Frac/pack techniques were widely implemented as a simultaneous solution for both sand control and production problems in highly permeable and unconsolidated formations.2–6 However, field experience indicates that some treatments do not perform as designed because of failure to achieve either tip screenout (TSO) or uncontrollable pressure growth. One of the challenges in treatment design and implementation is to accurately predict fluid leakoff in order to terminate fracture propagation at the desirable fracture length.7,8 However, prediction of the fluid leakoff is difficult because viscous fluids invade the formation and subsequently influence formation rock and fluid properties.9 Recent experiments also showed that spurt loss is a dominant leakoff phenomenon when the formation permeability is over 400 md.10,11 Therefore, the traditional model that does not account for spurt loss and fracture pressure is inadequate for modeling frac/pack applications in high-permeability formations.12–14 Recently, Fan and Economides introduced a leakoff model that predicts the transient flow of polymer solution displacing a reservoir fluid with a moving interface.15 The leakoff is pressure-dependent, with consideration of the pressure profile from the fracture face to the reservoir. In this paper, a frac/pack model with a pressure-dependent leakoff option is presented to study the interactions among the fluid leakoff, net pressure, and fracture dimensions for the Duri reservoir. The effects of fluid viscosity, reservoir mobility, and elastic modulus on the fluid efficiency and fracture net pressure are discussed. Field data are analyzed to demonstrate the model and its applications. Several aspects of treatment designs concerning selection of fracturing fluids, proppants, and treatment size are also discussed in this paper. Well-Completion Procedure in the Duri Field The bottomhole assembly for frac/pack treatment used in the Duri field is presented in Fig. 1. The completion event sequence is as follows.Rig-up and perforate intervals, with 180 phasing, 6 shots/ft, and 1.08-in.-diameter perforations.Wash perforations with KCl water between isolation cups.Run in hole with 6 5/8-in. wire wrap screen liner.Set modified fracture crossover with stinger.Perform minifracture on target intervals.Pump the frac/pack treatment per design.Pull fracture assembly out of hole.Set lead seal and place on production.

A critical review on analysis of sand producing and sand-control technologies for oil well in oilfields

“…In unconsolidated sandstone reservoirs with high permeability, increasing fracture conductivity is more beneficial to increase production than increasing fracture length. The percolation belt with high permeability can make the radial flow direction of fluid oil quasi-linear flow change, which can effectively reduce the production pressure difference, reduce the seepage resistance of crude oil, improve the flow conditions of oil and gas, so as to achieve the purpose of increasing production and sand control (Fan et al, 2001).…”