Optimizing Stimulation Treatments of Gravel Packed Wells by Analysis of Back Produced Fluids After Stimulation
Authors: Schmidt Terje , Haugland Torstein , Thingvoll Jan Tuxen 
Published: December 2007

SPE 31077 Optimizing Stimulation Treatments of Gravel Packed Wells by Analysis of Back Produced Fluids After Stimulation Terje Schmidt, SPE, Statoil, Torstein Haugland, SPE, Statoil, Jan Tuxen Thingvoll, Vestlandsforsking Copyright 1996, Society of Petroleum Engineers, Inc.

Abstract
A significant loss of productivity was observed in some of the first high rate gravel packed wells on the Gullfaks field. Productivity was initially restored by stimulation with clay acid (HBF4) and mud acid (HF/HCl) to remove and dissolve formation damage, believed to be caused by fines and clay. However, productivity soon decreased and frequent treatments with diluted HCl were necessary to maintain production from these wells.
The paper discusses mechanisms that can contribute to the observed permeability reductions following shortly after each treatment.
Evaluation of the HCl treatments revealed that large amounts of "AlF", "FeF" and CaF2 compounds, had formed as a result of the initial clay acid treatments. In one well, significant amounts of re-precipitated compounds of Al and Fe that could be attributed to the first acidizing job were detected up to 2 years later. Finally, the paper discusses how the treatments can be optimized based on the resulting increase in production rates and the chemical pattern revealed after stimulation

Introduction
The Gullfaks Field is located in the central part of the East Shetland Basin in the Northern North Sea (Fig. 1). The field is operated by Statoil on behalf of the Production License 050 consisting of Statoil (85%), Norsk Hydro (9%) and Saga Petroleum (6%). The field is developed with 3 Condeep platforms. Gullfaks A, B and C, and has been on production since December 1986. Current production is approximately 90 000 Sm3/D from 70 production wells, and approximately 60% of the 280*106 Sm3 recoverable reserves are produced. Estimated production life for the field, based on own reserves, is 20 years with peripheral water injection as the main drive mechanism.
The oil is located within three major sandstone units. the Brent Group, the Cook Formation and the Statfjord Formation. The Lower Brent, comprising of the Broom, Rannoch and Etive Formations represents a prograding delta front. The upper Brent is comprised of the Ness and Tarbert formations. The Ness Formation represents a delta top sequence with minor mouth bars dominating, while the Tarbert Formation represents the retrieving delta front building a homogenous sandstone unit on the top.
The reservoirs are overpressured, with an initial reservoir pressure of 310 bar at datum depth (1850 m below mean sea level), and a temperature of around 70 C. The oil is undersaturated with a saturation pressure around 245 bar varying with depth and location.
All Gullfaks formations contain poorly consolidated sands with high porosity and a potential need for sand control. Until May 1989, sand strength prediction and selective perforation was the main strategy and considerable research was conducted to avoid sand production. In a number of wells, water break through caused severe temporary reduction of maximum sand-free rate, and the need for sand control was clearly realized.
Since then, remedial action for sand control has been implemented in a total of 47 wells. Out of these, 30 are cased hole gravel packs and two are open hole completions (Fig. 2). With the exception of the first wells to be gravel packed, the gravel packed wells have performed very well, with no significant productivity decrease before sea water breakthrough and scaling problems occurred. This paper will concentrate on the experience with some of the relatively few wells that have needed regular acid treatments to maintain productivity; A-16A, A-23 and B-9 and where loss of productivity was not caused by scale. P. 41