Evaluation Of Fluoboric Acid Treatment In The Grand Isle Offshore Area Using Multiple Rate Flow Test
Authors: McBride J.R. , Rathbone M.J. , Thomas R.L. 
Published: December 2007

Consistent field results indicate fluoboric acid treatment effectively removes and prevents subsequent damage caused by migrating formation fines in gas wells located offshore Louisiana. Wells completed with inside casing gravel packs sometimes show rapid production decline following conventional mud acid production decline following conventional mud acid treatments. This decline is often attributed to formation fines that migrate into and restrict flow through gravel-packed perforations. Fluoboric acid, a retarded mud acid system, has been successfully used to treat completions with partially plugged perforations. Treatment with fluoboric acid not only perforations. Treatment with fluoboric acid not only provides deep hydrofluoric acid penetration but also provides deep hydrofluoric acid penetration but also stabilizes formation fines by fusing them together.

Multiple-rate flow tests were used to determine completions having damaged perforations. These completions were treated with conventional mud acid, resulting in only a temporary increase in production. Subsequently, the completions were treated with a fluoboric acid system. Multiple-rate flow tests were again used to study the effectiveness of the fluoboric acid treatment. Detailed case histories are presented along with a thorough explanation of evaluation techniques. Results are used to support previous laboratory tests.

Introduction

Sandstone matrix acidizing with conventional mud acid (hydrochloric-hydrofluoric acid mixtures) has long been used as a means of improving well productivity by removing formation damage and increasing permeability of the zone immediately around the permeability of the zone immediately around the wellbore. Laboratory and field studies have demonstrated the effectiveness of this type treatment. Although many wells show a good initial response to Mud Acid (MA) treatment, they later experience an unusually rapid decline in production rate. This decline is often attributed to plugging by migratory clays and other fines.

Muecke reports fines are present in all sandstone formations, whether deposited in the formation during geological structuring or introduced during drilling or completion operations. These fines are individual particles located on the interior surfaces of porous media and are not physically held in place by natural cementitious materials that hold larger sand grains together. Therefore, fines are free to migrate through the pores as fluids flow in the reservoir. If fines concentrate at a pore restriction, such as in a gravel-packed perforation, severe plugging can occur and result in large permeability plugging can occur and result in large permeability reductions. Both Saucier and Sparlin have reported over 50 percent reduction in gravel-pack permeability resulting from formation particles entering the gravel-pack sand.

Various clay stabilizing agents have been applied in an effort to control movement of clay formation fines. However, analyses of core samples from Gulf Coast wells show clay fines make up only a relatively small percentage of the total amount of fines found in formations. The dominant species was found to be quartz in the form of small sand grains. Other fines found were noncrystalline structured materials, feldspar, muscovite, calcite, dolomite and barite. Therefore, an ideal treatment designed to overcome the detrimental effects of fines migration must be capable of treating not only clay fines but all formation fines.

A new treatment using fluoboric acid (HBF4) has proven extremely effective for matrix acidizing of sandstone formations. The HBF4 is applied as an overflush to mud acid. Upon entering the formation, the HBF4 slowly hydrolyzes to generate hydroflouric acid (HF) providing deep, live acid penetration. Also, as a result of the treatment, any undissolved fines are stabilized. In contrast to conventional clay stabilizers, which act by ion exchange or adsorption, HBF, produces an actual chemical fusion of fines and clay platelets. This fusion process occurs as the result of borosilicate formation when the HBF, system reacts with siliceous minerals. Since fines are physically cemented in place, they are less likely to be disturbed by increased fluid flow following the acid treatment.