SPE/IADC 62792
Large-Scale Comparative Study of Prepacked Screen Cleanup using Acid and Enzyme Breaker

Mahmoud Asadi, SPE and Glenn S. Penny, SPE, Stim-Lab, Inc.

Copyright 2000, IADC/SPE Asia Pacific Drilling Technology

This paper was prepared for presentation at the 2000 IADC/SPE Asia Pacific Drilling Technology held in Kuala Lumpur, Malaysia, 11-13 September 2000.

This paper was selected for presentation by an IADC/SPE Program Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the International Association of Drilling Contractors or the Society of Petroleum Engineers and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the IADC or SPE, their officers, or members. Papers presented at the IADC/SPE meetings are subject to publication review by Editorial Committees of the IADC and SPE. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of where and by whom the paper was presented. Write Librarian, SPE, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435.

Abstract

Some of the completion failures are due to screen plugging. A number of remedies have been adopted by the industry to cleanup plugged screens. However, these cleanup methods are not investigated under downhole conditions. This study utilized a large-scale radial flow-loop to study prepacked screen cleanup under downhole conditions. First, the original permeability of each screen was measured with 10 ppg NaCl brine solution. A sized carbonate drill-in-fluid was then formulated and pumped for 3 hours and at a constant differential pressure of 60 psi across the screen, simulating a screen run in a wellbore. Inlet and outlet samples were collected for size distribution analysis and removal efficiency calculations of each screen. Each plugged screen was then subjected to a cleanup method such as acidizing, use of enzyme breaker, backflow, or a combination of these methods. Following cleanup, the permeability of each treated screen was measured with 10-ppg NaCl brine solution and compared to the screen's original permeability. The degree of plugging and percent area open to flow was evaluated based on the retained permeability of each screen.

Introduction

In horizontal wells, drilling mud is usually not displaced with a clean brine solution because of the costs. This requires screens to be run into mud, which contains solids. As a result, screens may plug and hence productivity declines. The overall screen plugging may occur due to the presence of various solids in the mud as well as migration of fines from the formation. Therefore, the degree of screen plugging depends on the solids used in the mud and formation fines. Prepacked screens are known to be more vulnerable to plugging due to the pore throat size of pack media.

A number of works has been conducted to investigate screen-plugging mechanisms and to evaluate various cleanup efficiencies. Some of these works have focused on the role of drill solids in plugging and drill-in-fluid optimization to reduce screen plugging. [1,2] Some works have been focusing on screen design and its proper selection to prevent plugging [3,4]. Others have been concentrating on cleanup evaluation of screens after they are plugged. [5,6,7] Also, screen plugging mechanisms and their remedies have been the subject study of some authors. [8,9,10] All of these research efforts are informative, but not comprehensive. As a result, more works are being done to fully understand the mechanism of screen plugging under downhole conditions and to evaluate various cleanup methods. These works consist of new screen designs to reduce plugging potential and increase in removal efficiency, development of new enzyme breakers that are more effective and cost efficient, and improved cleanup methods which are more environmentally safe.

The objective of this study was to evaluate the efficiency of various cleanup methods using prepacked screens, which are more susceptible to plugging.

SPE 64413
Sand Control Screen Plugging and Cleanup

Mahmoud Asadi, SPE, and Glenn S. Penny, SPE, Stim-Lab., Inc.

Copyright 2000, Society of Petroleum Engineers Inc.

This paper was prepared for presentation at the 2000 SPE Asia Pacific Oil and Gas Conference and Exhibition held in Brisbane, Australia, 16-18 October 2000.

This paper was selected for presentation by an SPE Program Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Papers presented at SPE meetings are subject to publication review by Editorial Committees of the Society of Petroleum Engineers. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of where and by whom the paper was presented. Write Librarian, SPE, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435.

Abstract

Screens are used to mechanically restrain formation solid entrance to a wellbore. As effective as this process may be, in many cases it fails to retain solids. This is due to screen plugging either during screen run in a wellbore or during production where larger size particles bridge against a screen and finer size particles plug the screen. In either case, plugged screens are subjected to a cleanup treatment. In this paper, a series of large-scale tests are conducted to evaluate screen cleanup methods such as acid spotting, use of enzyme and acid activated-breaker, backflow, and a combination of these methods under downhole conditions. At first, each screen's retention performance is evaluated. Then, a drill-in-fluid containing solids is pumped into a large-scale flow-cell at a constant pressure drop across the screen to simulate a screen run down in a wellbore. Inlet and outlet samples are collected to evaluate removal efficiency of each screen. Then, each plugged screen is subjected to a cleanup method. The effectiveness of each cleanup technique is evaluated based on the retained permeability of each screen. Degree of plugging and percent area open to flow for each screen is also presented.

Introduction

All mechanical techniques used in controlling sand production are based on the bridging theory. This principle is based on the large size particles bridging against a mechanical filtration device such as a screen that allows the flow of fluid and restrains solids. In many cases, these techniques are not successful. In screen completion, the failure is due to either screens being run into dirty drill-in-fluids, which will cause screen plugging, or when screen is positioned in a wellbore, larger solids bridge against the screen and finer materials will either pass through the screen and onto the wellbore or plug the screen. [1,2] If finer particles pass through the screen, the problem of local erosion becomes important. [2] In addition, sand production will be an issue. If these fine particles are held by screen, they will eventually plug the screen. Therefore, size selection of solids present in a mud becomes very important because solids must bridge across pore throat properly to form an effective filtercake. [4]

Today, various techniques are used to plug and clean screens. Most of the previous works are done on small-scale disk size screen. The cleanup techniques include acidizing under dynamic conditions, acid spotting, use of enzyme breaker, and oxidizer. This study utilizes a large-scale flow-loop to plug screens and then uses various screen cleanup methods under downhole conditions.

Experimental Setup

Laboratory Flow-Loop - A 6-ft long by 6-in. inside diameter high strength Plexiglas flow-cell model was utilized to evaluate screen plugging and various cleanup efficiencies under downhole conditions. The flow-cell model is capable of operating at 300 psi pressure differential and 180°F. The model allows visualization of screen plugging and cleanups. The active screen length used in this study was about 5 ft. One end of each screen was capped, and a 6 in. base pipe was connected to the other end to discharge fluid entering into the screen during a test. To prevent any turbulence effect, a flow-diverter was placed onto the capped end of the screen where fluid was entering the flow cell. As fluid entered the flow cell, it flowed around the screen, simulating a screen run down a wellbore, and was then flowed out of the flow cell through the 6 in. base pipe. Pressure drop across the screen was measured between the inlet point, before fluid entered the flow cell, and at the point where fluid exited the flow cell through the 6 in. blank pipe. Two valves, one at the inlet and one at the outlet, were installed to collect inlet and outlet samples for size distribution analysis of solids in the formulated drill-in-fluid. These data were used to evaluate screen retention capabilities. A Heat tape was wrapped around the insulated flow cell to maintain a constant temperature during acid spotting and or enzyme treatment. Flow rate, pressure drop, and fluid temperature data were electronically collected. Fig. 1 shows schematic of the experimental setup.