Reduced Well Spacing Combined With Polymer Flooding Improves Oil Recovery From Marginal Reservoirs
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 96946, "Combining Small Well Spacing With Polymer Flooding To Improve Oil Recovery of Marginal Reservoirs," by J. Cheng, D. Wang, X. Sui, SPE, H. Zeng, and W. Bai, SPE, Daqing Oilfield Co. Ltd., prepared for the 2006 SPE/DOE Symposium on Improved Oil Recovery, Tulsa, 22,–26 April.
The full-length paper details laboratory studies of polymer-injection parameters and project-design optimization. On the basis of these studies, a pilot test combining close well spacing and polymer flooding was undertaken. Well spacing was 100 m, zone thickness was less than 1 m, and permeability was 5 to 10 md. Both laboratory studies and pilot results show increased volumetric sweep efficiency. The pilot test indicates that combining infill wells with polymer flooding is economically feasible with 10% original-oil-in-place incremental recovery at a production cost of U.S., $10/bbl.

Introduction
In continental, multilayer, heterogeneous sandstone oil fields, some reservoirs with poor connectivity and low permeability have low recovery factors. The low degree of layer connection and inner-layer interferences lead to poor waterflooding efficiency. To improve oil recovery and increase recoverable reserves, infill wells were drilled to improve reservoir connectivity and a polymer solution was injected. Average incremental oil per day for a single well was 1.83 times original production. Water cut decreased 10.2%, and oil recovery increased more than 10%.

Pilot Area
The pilot-test area was an inverted-five-spot well pattern with 25 wells—nine injectors and 16 producers (Fig. 1). Distance between injectors and producers was 100 m. Target zones comprise the marginal Sa and Pu II reservoirs. Polymer flooding focused on layers with permeability of 100 md or less that account for 84% of the total layers. Layers with an effective thickness of less than 1 m account for 90% of the total.

Laboratory
Core studies show that the lower the permeability, the lower the molecular weight (MW) of polymer required. Because the target reservoirs have low permeabilities and small pore radius, appropriate polymer MW must be chosen on the basis of reservoir properties. Laboratory studies determined the upper limit of polymer MW on the basis of reservoir permeability. Polymers with a 2.4×106 MW satisfy the needs of reservoirs with a 20×10−3-µm2 permeability, while 5.5×106 MW is suit -able for reservoirs with a 50×10−3-µm2 permeability, and 10×106 MW is suitable for reservoirs with a 200×10−3-µm2 permeability.

Polymer-Injection Trial
Before the pilot test, one well was selected for a trial to determine the injection parameters for the entire test area. Polymer with a 7×106 MW was injected at a concentration of 600 mg/L in the first injection well. The injection process worked very well at an 8.0-MPa injection pressure. No injection-pressure buildup was observed for any of the layers, indicating that all had some injectivity. Gradual adjustments were made as a result of the actual injection conditions. Injection viscosity at the wellhead reached 12 to 17 mPa.s, and the injection rate was 0.25 to 0.33 pore volumes (PVs)/a. Polymer-injection volume was determined by the actual production conditions.