Successful Acid Treatments in Horizontal Openholes using Dynamic Diversion and Instant Response Downhole Mixing—An In-Depth Postjob Evaluation

Abstract

This paper discusses a relatively new acid-stimulation process that uses dynamic fluid energy to divert flow into a specific fracture point in the well that can initiate and accurately place a hydraulic fracture. These acid-stimulation methods often use two independent fluid streams: the acid phase down the treating string and other liquids or gases down the annulus. The annular fluid may simply serve as a wellbore pressurizing component, or with this process, two different fluids can be mixed downhole with high energy to form a homogenous mixture. Using variations of this process, treatments were performed in four openhole, horizontal wells in the same formation. The first well was acid-fractured with coiled tubing to place many small fractures along the open hole. The second well was acidfractured with coiled tubing, but downhole mixing concepts were also used to provide in-situ generation of CO2 foam where fewer, but larger, fractures were placed in this well. The third and fourth wells were treated with acid using blasting-type rotating or non-rotating jetting tools while mixing the acid with CO2 downhole. Numerous small, near-wellbore fractures were the expected result in these two wells. Experimental laboratory investigation of the physical mechanisms that contributed to these successful stimulations was considered important. Large-scale laboratory tests were performed to more closely examine the physical rock response to the jetting mechanism and conditions. The findings of these tests are presented in this paper. Introduction In an openhole lateral, successful production enhancement treatment depends on identifying the causes of the production deficiency for the specific well to be treated, properly understanding the remedial treatment options available, and evaluating the economics of available options. Assuming the problem is not inadequate reservoir size or recoverable reserves, production deficiencies can result from many factors in a horizontal openhole completion, including: wellbore debris, filter cake or other near-wellbore damage, deeper permeability damage, low formation permeability, or even wellbore location within the reservoir. If the production problem is caused by debris plugging along the lateral, the well can usually be cleaned with an effective hydrablasting and cleaning process. This service generally combines specialized fluid systems (often foams) and possibly a wash tool designed for the different mechanics associated with cleaning deviated or horizontal wellbores. Some processes incorporate computerized design software to properly plan the clean-out operation. If a solids filter-cake has caused near-wellbore damage that is responsible for the production deficiency, an acid wash might be the best solution. Coiled tubing (CT) applications can often allow more efficient washing of the entire lateral or of specific preselected zones. Using tools with jetting nozzles can enhance the effectiveness of acid washes, or even nonreactive fluids that provide the benefits of only mechanical-jetting effects. If deeper damage in the near-wellbore region is decreasing production, an operator may desire to place several small hydraulic fractures at many places along the lateral section to bypass the damage. Such small fractures can use relatively small volumes of acid to give a beneficial result similar to increasing the wellbore diameter, and do so without problems associated with excessive hole enlargement. If the wellbore is located in a poorly producing zone in the reservoir some reasonable distance from a better part of the reservoir, or if a vertical permeability barrier exists, the operator may need to create larger fractures that can communicate the wellbore with more productive zones.

Consideration for Future Stimulation Options Is Vital in Deciding Horizontal Well Drilling and Completion Schemes for Production Optimization

“…One known solution is hydrajetfracturing technology, which was created for transverse fracture placement in horizontal wells. This technology has been published many times, and therefore is not discussed in depth (McDaniel and Willett 2002;Rees et al 2001;Surjaatmadja et al 2002). Unlike other techniques that use conventional packers or bridge plugs (Meyerhofer et al 2000;Warpinski et al 2001), this technology uses dynamic sealing to "seal" flow into the present fracture instead of reopening previous frac locations.…”

Stimulation in Asia Pacific Region—Challenges and Opportunities

“…During appraisal stage, Shell performed hydraulic fracturing operation on vertical wells with success 6 under a significantly different stimulation service infrastructure in China 7 . During technology selection exercise, multiple hydraulic fracturing in open hole horizontal wells was investigated [8][9][10] . In theory, such technology could work, however, it would not bring economic benefit to the project.…”

“…They all have proven track record of success. In terms of acid systems, they vary from self-diverting surfactant acid system [10][11][12] to insitu crosslinked gelled acid system [14][15][16] . From engineering point of view, the key factor of success in pumping any of the proposed acid systems is to effectively diverse the acid to cover the whole section of the well and have enough pressure to drive the acid into the formation to generate wormholes.…”

Efficient Coiled-Tubing Fracturing for Proppant-Placement Assurance and Contingency-Cost Mitigation

“…The use of gases, such as N 2 or CO 2, to form a foam or commingled fluid mixtures has been successful in the industry (Surjaatmadja et al 2002). These gases are usually pumped at near-ambient temperatures and are mixed with known stimulation liquids to perform the treatment with less liquid invasion into the formation porosity.…”