Massive Annular Fracturing Practices in BJC Gas Field, Sichuan, China

Abstract

Massive hydraulic fracturing has been successfully applied in tight gas reservoir development. Economic completion of tight gas sands with large hydraulic fracturing treatments requires cost effective and time saving operations. Traditional large fracturing jobs are usually pumped down 5.5" or 4.5" casing to meet the requirement of high pumping rate (30~55bpm). Post-frac snubbing operations are often needed to run tubing and clean out wellbores. Snubbing operations can be costly in terms of investment and time. Annular fracs have been applied in the industry as an alternative completion strategy. However, previously documented annular jobs have been small size, ranging from 40k to 200k lbs of proppant pumped at relatively low injection rates of 15–25 BPM. This paper describes the practices of massive annular fracturing treatments down the 5–1/2" by 2–3/8" annulus used at the Bajiaochang Gas Field, Sichuan Basin, China, as a substitute to fracturing down casing and subsequent snubbing operations. Three treatments have been performed since October 2005. The first job had to be terminated with 70% of the designed proppant (394k lbs) pumped because of the failure of the blast joint. Lessons learned were outlined and modifications were made to the blast joint and wellhead. Subsequent treatments were performed without mechanical failures with 350k and 282k lbs of proppant pumped at 30 to 35 bpm injection rates. The completion cycle time was reduced about 20% with substantial savings of up to $260k in well completion costs. Improved monitoring of bottom hole pressure from static tubing for 3D fracture modeling and effective treatment evaluation were also a benefit. This data has aided fracture design in highly complex fracture stimulation applications. Additional advantages also include: easily circulating out proppant if screen outs occur and more efficient flow back for lower rate wells. Introduction: A tight gas sand exploration and development program has been on going for several years in Chuanzhong Block, Sichuan Basin, China. The reservoir is a fluvial deposit which is located in a slight thrust-fault environment with a possible small strike-slip component. It is over-pressured with micro-Darcy permeability sand (See Figure 1–2). Fifteen wells have been drilled by the operator. All wells are completed in the gas-bearing XX-4 formation at depths of approximately 3150m MD (about 3000m TVD, "S" shape wells are drilled from multi-well pads) by utilizing massive hydraulic fracturing treatments that are the largest and the most complicated in China. The general completion practice at Bajiaochang had been traditional cross-linked gel fracs pumped down 5–1/2" casing at rates from 40 - 50 BPM. Other frac designs including slick water treatment and hybrid fracs have also been used. Due to the potential for waterblocks, post-frac snubbing operations were performed to run production tubing and wash out sand while maintaining an underbalanced wellbore condition. Generally, the completion cycle time was around 41days before handing well over to production. The majority of the fracturing treatments were designed by using the 3D Mfrac model. Real time data has been monitored by linking-up a laptop to the treatment van computer for fracture evaluation and re-design on-site. Lack of bottom hole pressure measurement makes it very difficult to estimate the complex fracture behavior in this high stress environment. Estimation from surface treating pressure, which includes variable friction, may lead to erroneous interpretations.