Single-Trip, Multiple-Zone Frac Packing Offshore Sand Control: Overview of 58 Case Histories

Abstract

Abstract Industry experience has shown that frac pack completions can provide effective production stimulation and reliable sand control for unconsolidated sand reservoirs. One of the more rewarding areas of opportunity is in the completion of longer and more complex intervals that can be stimulated with a single frac pack, high-rate water-pack (HRWP) or EX-Tension PacSM Service treatment. One of the major considerations in planning the \well completions in the BZ-25–1 field was design efficiency. To complete each individually zone independently with a Stack Pack completion would have been very time consuming. This paper presents the results from installing frac-pack and HRWP treatments through a single-trip, multiple-zone (STMZ) completion system in the offshore Bohai field in China. Using the 9 5/8 × 4.75-in. ID single-trip gravel pack tools, operators have performed frac packing and HRWP treatments on 58 wells, on three platforms, during 2003 and 2005 with an average of 3 to 5 zones per well, spending an average of less than 3 days to complete each well. This paper presents the selection process for the completion technique used to achieve high production efficiency and initial performance of the field. It provides guidelines and results associated with frac packing and demonstrates the importance of selecting completion techniques that will provide high-efficiency production. Introduction The BZ-25–1 field (Fig. 1) is located in Bohai Bay, 150 km from the offshore base in Tanggu. The phase I development consisted of three platforms (B, D, and E) with a total of 83 wells. The first well was drilled in May 2003, was completed in August of 2003, and the first oil was produced in August 2004. There have been no workover events. There are 15 wells with 2 zones, 21 wells with 3 zones, 10 wells with 4 zones, and 4 wells with 5 zones. All wells require sand control. Among them, 61 intervals were completed using frac pack completion techniques (Fig. 2) with a low-gel borate (LGB) fluid system with a tip screen-out (TSO) design strategy; 31 intervals were completed using high-rate circulating gravel pack completion techniques (Fig. 3), and 17 intervals were completed using Ex-Tension PacSM Service completions (Fig. 4) with low sand concentration and with linear gel fluid. The remaining horizontal wells were completed as openhole with screens alone (no gravel packing), and the injection wells were completed in cased hole with screens. All production was ESP assisted. Reservoir Properties The reservoirs are multilayered thin channel sand deposits (Fig. 5) that are unconsolidated. This is an oil reservoir and the oil beds are mainly distributed in Nm IV-V oil groups. It is a high-porosity high-permeability reservoir bearing heavy crude with oil gravity ranging from 12 to 18 API. The porosity of reservoirs in the Lower Ming section is mainly distributed between 17.8% and 40%; average porosity is 34%. Permeability is 300 mD to 6000 mD---average permeability is 1750 mD. Bottomhole temperature is 150°F-180°F. Reservoir TVD is 1650 m-1850 m. Bottomhole pressure is 1500 psi-2800 psi Frac Pack TSO frac pack (Fig. 2) design strategy was applied where possible. When the reservoir was close to a water or gas zone and the cement quality is poor, the HRWPtechniques (Fig. 3) were used. In cases where the permeability was very high and required fluid viscosity to create fracture, linear gel fluid was used (Fig. 4). The benefits of frac packing are well documented.1–22 Frac pack completions have been about twice as effective as gravel packs in optimizing deliverability and production.2,5,14,16 In addition to improving skins and accelerating recovery of reserves, these completions have lower pressure drops, lower drawdown, and lower turbulent flow skin than nonfrac packed wells.2,3,5,6–9,12–14,16–19,21–25