Abstract

Tortuousity is the creation of small fractures leading from perforations that may or may not fully connect to the main fracture during hydraulic fracture operations. The lack of communication and friction associated with the tortuous paths from these small fractures creates a pressure drop at the near wellbore that ultimately reduces the main fracture width, height, and length. Although tortuousity is typically observed in deviated wells, tortuousity effects have been observed in both deviated and vertical wells in the Tanjung field. Failures to control the effects of tortuousity have resulted in premature screen outs, and failure to stimulate the desired interval. This paper will describe the methods used to identify tortuousity, probable causes for both vertical and deviated wells, the methods used to combat these effects, and the results of applying the new methods used in the Tanjung field.

The Tanjung field is a mature oilfield that operates 74 producing wells under secondary recovery. Hydraulic fracturing is a major method of stimulation to increase drainage areas and combat near wellbore damage in Tanjung wells. Over 165 hydraulic fracturing operations have been performed on wells in the Tanjung field since 1952. More than 80 of these treatments were performed in 2002 to August 2003, and are the main reason for maintaining flat production in the field with a 33% average decline rate in base production.

To combat the near wellbore frictional effects, many changes were made including: 1) Optimizing perforation design including phase, the number of perforations and interval lengths, 2) Optimizing treatment pumping rates, 3) Maintaining proper frac fluid rheology throughout the treatment, and 4) By modifying the sand ramp. Applying these techniques has resulted in being able to successfully stimulate wells that in the past would have screened out prematurely.

Although a very successful fracture campaign, some wells screened out prematurely with similar treatments and in what was thought to be similar reservoirs geologically. Analysis of the wellbore, frac and geological data suggest the near wellbore effects are related to both well deviation, amount of conglomerate present, and completion design. Down hole pressure gauges are used to measure actual bottom hole pressure during data fracs on selected wells. By doing so, sources of error in calculating bottom hole pressure by assuming friction values have been eliminated, and better resolution of near wellbore effects can be observed and quantified. Data fracs include step up and step down test, and mini fracs. The tests provide very useful information including fracture extension pressures, perforation friction, and amount of tortuousity present.