Proposal

Numerous oil-producing wells in Southern Oman are completed with wire wrap screens (WWS), internal gravel packs (IGPs), and predrilled liners. These wells produce from mature clastic formations where fines migration and subsequent blockage of screens can result in impaired oil production. In the past, conventional treatment using coiled tubing and a jetting tool has been chosen to remove this damage. The gains resulting from these intervention activities were more often than not short-lived. This lack of longevity required frequent well intervention and oil deferment, often resulting in a loss of revenue.

Recently, a systematic approach was undertaken to evaluate the wellbore cleaning and stimulation tools that are currently available in the industry. This approach was implemented as a trial of three cleanout tools in oil-producing wells. This paper describes the results of using these tools for cleanout and stimulation of sand screens.

Excellent success was achieved with a pulse-jetting tool operating on the Principles of Coanda effect. This tool is further described in the following sections, and results of its use in a number of oil-producing wells are presented. The effect of the cleanout procedure is presented in terms of initial production and sustainment of production level.

This paper also outlines the importance of using proper cleaning and/or stimulation fluid. To help avoid clay-swelling problems, special emphasis is placed on the brine fluid salt concentration.

Based on the success it has achieved, the pulse-jetting tool is now a standard tool used in well interventions geared for wellbore cleanout and/or stimulation. The versatility of the tool enables it to be deployed for use with coiled tubing or regular workover strings. Many fluids, including nitrogen, can be pumped through the tool. This versatility is important because most of the wells are sub-hydrostatic and require the use of nitrified fluid to maintain circulation in case cleanout and well-lifting operations occur immediately after sandstone acid stimulation.

Introduction

Screens are installed in wells to prevent formation sand from being produced along with the oil. These screens, however, are prone to plugging, which in turn leads to reduced production. This problem is currently solved by bullheading brine into the annulus, which flushes the sand off the screen face when coiled tubing units are not available. Although this treatment is cheap in restoring some of the productivity, it is ineffective with short-lived gains, very localized, and needs to be repeated every three months. Frequent application of bullhead treatments can increase the risk of formation impairment and associated reduction in production.

To help avoid these problems, three cleaning tools from different service companies were used and evaluated: a rotational-cavitation jetting tool from Service Company A, a piezo-electric sonic tool (PST) from Service Company B, and a pulse-jetting tool from Service Company C.

Results of the Trials
Rotational-Cavitation Tool

Trial runs of the tool were completed in 2001. Nine jobs were performed. In general, the initial cleanup of WWS generated oil gain and raised the fluid level for the wells. The increased production level was not sustained for more than one month in some cases, and after three months, all the wells had dropped in gross production.

Piezo-Electric Sonic Tool

Trial runs of the PST acoustic tool were completed in October 2002. Five jobs were performed. In general, the initial cleanups of gravel pack (GP) WWS generated oil gain and raised the fluid level for the wells (gross increased). The increased production level in three of the wells was not sustained for more than one month, and in the other two cases, no improvement in production was observed.

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