During mid-1992, a remedial workover program was implemented offshore within the Gulf of Mexico (GOM) on low rate marginal reserve sand producing wellbores. A process referred to as thru-tubing gravel packing (TTGP) was utilized on five offshore wells along with one gulf coast land well. This technique involved the placement of ceramic beads and pre-packed screen within existing completions, using coiled tubing in order to eliminate sand production. (Fig. 1) The basic steps of the thru-tubing wash down gravel pack consist of wellbore cleanup, coiled tubing counter correlation, injectivity testing, running a dummy assembly, ceramic bead placement, prepacked screen placement, and installation of an isolation device. The TTGP method demonstrated versatility in being able to adapt to individual well conditions. This remedial sand control technique was successfully applied in highly deviated wellbores, upper zones of a dual completion, and within existing gravel packs for screen repair.
TTGP's were performed offshore in as little as five days with a coiled tubing unit for a cost of $63,500. Five wells were mechanically successful, while only three wells were considered economic successes. Simple individual payouts for these wells ranged from 46 to 77 days, with an associated production gain of 2020 MCFPD and 39 BOPD. Economic success improved over time as mechanical success increased. Project incremental production was 1,070 MCFPD and 38 BOPD. Total cost of the project was $571,807. Average offshore workover cost was $110,761 while the workover cost for the onshore well was $18,000.
Throughout the implementation of this program, mechanical problems were addressed which led to improvements in subsequent workover programs. This paper details the TTGP program and recommends mechanical improvements for performing future thru-tubing gravel packs.
Effective sand control has long been a concern within unconsolidated sandstone formations. The primary method currently used for sand control is gravel packing the completion. This method requires a conventional rig and is the most expensive alternative. Secondary processes are less expensive that normally are completed thru-tubing with the use of wireline, coiled tubing, or snubbing units. These include such things as resin applications, prepacked screen installations, and thru-tubing gravel packing. Secondary methods have additional mechanical risks due to the use of smaller tools in older wellbores.
Candidates for remedial sand control were normally marginal wells with low reserves and sand production. The reserves couldn't support higher workover costs associated with a conventional workover rig while sand restricted hydrocarbon production.