Abstract

In the reaction of formation rock with acid, as in a matrix acidizing treatment, varying quantities of insoluble fines are released or otherwise dislocated by the flow. Normal reservoir flow can also cause the breaking or dislodging of fragile clays and minerals present in the pore spaces. U allowed to settle and accumulate, these fines can plug pore throats. The addition of a silt-suspending additive to the acid used for a matrix acidizing treatment will help prevent this accumulation by keeping the fines distributed in the fluid. This prevents the plugging that can occur while removing these fines from the formation.

The part silt-suspending additives play in both preventing and removing formation damage may be broken down into three areas of discussion: causes of production declines, factors to consider in designing the proper acidizing treatment, and perhaps most important, laboratory results obtained when testing silt-suspending additives. These test results include measurements of suspending power and surface tension, with emphasis on the analysis of three formation cores and their response to treatment with the silt-suspending acid mixture. The cores under evaluation include:

  1. Basal Colorado Formation – Cessford field(915.7-915.9 meters).

  2. Manville Formation – Cessford field (1010.1-1010.3 meters).

  3. Glauconite Formation – Murphy field (1010.1-1046.0 meters).

Introduction

Matrix acidizing sandstone for the removal of silts and fines is a commonly practiced completion and workover treatment.1–3 In the removal of these solids, surfactants and acid blends have been tested in many areas. Because no area is exactly the same as another, no single additive is universally applicable. By selectively blending surfactants, it is possible to obtain a mixture with more universal properties.

During the dissolution of formation rock by hydrochloric acid or hydrofluoric acid treatments, the majority of the producing formations release some fines. Insoluble materials found in formations, such as clays, feldspars and silicas, are released in large amounts. These solids must be removed to obtain any conductivity in etched channels or pores.

DECLINES IN PRODUCTION

A decline in production can be the result of one or a combination of factors. An obvious reason for production drop-off is a depleted reservoir. Secondly, however, the problem may be reduced conductivity in the area immediately adjacent to the wellbore. This reduction in conductivity or permeability can occur in either the formation matrix or in a proppant packed fracture. In either case, permeability reduction, whether due to drilling fluids, silts and fines migration, or the precipitation of insoluble compounds, is a result of flow restrictions.

The impact of this permeability loss on well production is more severe as it nears the wellbore. In this situation, the formation matrix acts as a filtering medium.

The following procedure was used in the analysis of the selected acid blends. A total of 10 acid solutions were prepared. Five of the solutions contained the test additives including the suspending agents the other five solutions were mixed as corresponding references, containing all the same additives except the suspending agent.

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