SPE 96965

Laboratory Studies of In-Situ Generated Acid to Remove Filter Cake in Gas Wells

H.A. Nasr-El-Din, SPE, M.B. Al-Otaibi, SPE, and A.A. Al-Qahtani, Saudi Aramco, and I.D. McKay, SPE, Cleansorb Ltd.

Paper presented at the SPE Annual Technical Conference and Exhibition, 9-12 October 2005, Dallas, Texas.  Copyright 2005, Society of Petroleum Engineers, Inc. (reproduction of abstract is permissible).

ABSTRACT

Many horizontal/multilateral wells are drilled using water-based mud. In general, drilling fluids consist of xanthan, starch or polyanionic cellulose with bridging agents like sized calcium carbonate and salt particulates.  During drilling and completion operations, it is essential to form an external filter cake to control fluid losses. However, effective filter cake removal during well completion is very difficult to achieve, especially in low permeability formations. Increasing use of long horizontal and multi-lateral wells with very low draw-down makes efficient cleaning and stimulation treatments even more challenging. Placement of corrosive acids to dissolve carbonate is problematic particularly in horizontal wells with extended reach. Polymer-breaking enzymes and oxidizers are effective at degrading polymer residues present in the wellbore, but will not attack carbonate or other acid-soluble materials. 

A new fluid addresses several key problems inherent in removal of drilling fluid filter cake, cleaning, and stimulation treatments of horizontal and multilateral wells. This fluid generates an organic acid (acetic or formic) in-situ, thus providing a very controlled acidising process for mud solids. The fluid can also incorporate enzymes to remove biopolymers. Several lab experiments were conducted to examine the compatibility of fluids with enzymes. The kinetics of acid producing reactions were examined in detail. A modified HPHT fluid loss cell was used to assess the effectiveness of the system in cleaning drill-in fluid damage. The surface tension of fresh and spent acid was measured as a function of temperature. 

Lab results indicated that the fluid is compatible with the enzyme system. Also, the fluid is non-corrosive and has minimum impact on well tubulars. Unlike conventional reactive acids such as HCl, the slow reaction kinetics ensure better distribution of the new fluid across long sections with minimal leak-off during pumping. The HPHT tests showed a significant improvement in the efficiency of removing filter cake. Surface tension measurements indicated that the spent acid solution had low surface tension, which will accelerate lifting these fluids from tight formations.

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