Dendritic-Acidizing Update: The Light at the End of the Tunnel
Abstract: Summary In this paper, we provide an update on recent advances for and summarize global experiences with dendritic-acidizing (DA) methods, or acid tunneling. We include both coiled-tubing (CT) deployed methods and non-CT methods, and discuss process limitations, candidate-selection criteria, job-design factors, operational learnings, risks, and surveillance requirements and opportunities. A comprehensive review of published information is provided for three different tunneling methods along with… Show more
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Cited by 3 publications
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Summary Acid tunneling is an acid-jetting method for stimulating carbonate reservoirs. Several case histories from around the world were presented in the past showing optimistic post-stimulation production increases in openhole wells compared with conventional coiled-tubing (CT) acid jetting, matrix acidizing, and acid fracturing. However, many questions about the actual tunnel creation and tunneling efficiency are still not answered. In this paper, the results of an innovative full-scale research program involving water and acid jetting are reported for the first time. The tunnels are constructed through chemical reaction and mechanical erosion by pumping hydrochloric acid (HCl) through conventional CT and a bottomhole assembly (BHA) with jetting nozzles and two pressure-activated bending joints that control the tunnel-initiation directions. If the jetting speed is too high and the acid is not consumed in front of the BHA during the main tunneling process, then unspent acid flows toward the back of the BHA and creates main wellbore and tunnel enlargement with potential wormholes as fluid leaks off, lowering the tunneling-length efficiency. Full-scale water- and acid-jetting tests were performed on Indiana limestone cores with 2- to 4-md permeability and 12 to 14% porosity, sourced from the same supplier. Many field-realistic combinations of nozzle sizes, jetting speeds, and casing pressures were included in the testing program. The cores were 3.75 in. in diameter × 6 in. in length for the water tests and 12 in. in diameter × 18 in. in length for the tests with 15-wt% HCl acid. The jetting BHA was moved as the tunnels were constructed, at constant force on the nozzle mole, to minimize the nozzle standoff. Six acid tests were performed at the ambient temperature of 46°F and two at 97°F. The results from the acid tests show that the acid-tunneling efficiency, defined as the tunnel length divided by the acid volume, can be optimized by reducing the nozzle size and pump rate. The results from the water and acid tests with exactly the same parameters to match the actual CT operations in the field show that the tunnels are constructed mostly by chemical reaction and not by mechanical erosion. The acid-tunneling efficiencies obtained from the full-scale acid tests are superior to the average tunneling efficiency of more than 500 actual tunnels constructed during more than 100 acid-tunneling operations performed to date worldwide. Although the tunnel lengths and acid volumes for the actual tunnels constructed during the previous acid-tunneling operations were recorded by the service company performing those operations, little downhole temperature and formation characterization data were provided by the operators to the service company. Thus, the downhole-temperature and formation-characterization effects on the acid-tunneling efficiency for the previous field operations are unknown. In this paper, we describe the full-scale water- and acid-jetting tests on Indiana limestone cores. The major novelty of this test program consists of performing all measurements with casing pressure, unlike all previous water- and acid-jetting studies performed at atmospheric conditions and reported in the literature, which is closer to the field conditions during CT operations. The novel understanding of the combined effect of the nozzle size, pump rate, and casing pressure significantly improves the actual acid-tunneling efficiency.
Summary Acid tunneling is an acid-jetting method for stimulating carbonate reservoirs. Several case histories from around the world were presented in the past showing optimistic post-stimulation production increases in openhole wells compared with conventional coiled-tubing (CT) acid jetting, matrix acidizing, and acid fracturing. However, many questions about the actual tunnel creation and tunneling efficiency are still not answered. In this paper, the results of an innovative full-scale research program involving water and acid jetting are reported for the first time. The tunnels are constructed through chemical reaction and mechanical erosion by pumping hydrochloric acid (HCl) through conventional CT and a bottomhole assembly (BHA) with jetting nozzles and two pressure-activated bending joints that control the tunnel-initiation directions. If the jetting speed is too high and the acid is not consumed in front of the BHA during the main tunneling process, then unspent acid flows toward the back of the BHA and creates main wellbore and tunnel enlargement with potential wormholes as fluid leaks off, lowering the tunneling-length efficiency. Full-scale water- and acid-jetting tests were performed on Indiana limestone cores with 2- to 4-md permeability and 12 to 14% porosity, sourced from the same supplier. Many field-realistic combinations of nozzle sizes, jetting speeds, and casing pressures were included in the testing program. The cores were 3.75 in. in diameter × 6 in. in length for the water tests and 12 in. in diameter × 18 in. in length for the tests with 15-wt% HCl acid. The jetting BHA was moved as the tunnels were constructed, at constant force on the nozzle mole, to minimize the nozzle standoff. Six acid tests were performed at the ambient temperature of 46°F and two at 97°F. The results from the acid tests show that the acid-tunneling efficiency, defined as the tunnel length divided by the acid volume, can be optimized by reducing the nozzle size and pump rate. The results from the water and acid tests with exactly the same parameters to match the actual CT operations in the field show that the tunnels are constructed mostly by chemical reaction and not by mechanical erosion. The acid-tunneling efficiencies obtained from the full-scale acid tests are superior to the average tunneling efficiency of more than 500 actual tunnels constructed during more than 100 acid-tunneling operations performed to date worldwide. Although the tunnel lengths and acid volumes for the actual tunnels constructed during the previous acid-tunneling operations were recorded by the service company performing those operations, little downhole temperature and formation characterization data were provided by the operators to the service company. Thus, the downhole-temperature and formation-characterization effects on the acid-tunneling efficiency for the previous field operations are unknown. In this paper, we describe the full-scale water- and acid-jetting tests on Indiana limestone cores. The major novelty of this test program consists of performing all measurements with casing pressure, unlike all previous water- and acid-jetting studies performed at atmospheric conditions and reported in the literature, which is closer to the field conditions during CT operations. The novel understanding of the combined effect of the nozzle size, pump rate, and casing pressure significantly improves the actual acid-tunneling efficiency.
The oil and gas industry has been developing various technologies to increase the productivity and recovery of hydrocarbons from conventional and unconventional reservoirs. Reservoir stimulation is an essential operation used to enhance production in many fields around the world. Hydraulic fracturing and acid treatments are the main stimulation methods. Reservoir tunneling concepts are used to drill branched channels in the formation from the main wellbore. With thousands of tunnels drilled to date, it is a viable technique that can improve the recovery of selected reservoirs. This paper reviews the recent developments in reservoir tunneling technologies and their current applications. These tunneling methods can be categorized mainly into water jetting, abrasive jetting, reactive jetting (acid), and needle and mechanical tunneling (radial drilling). The paper includes reviewing and analyzing these techniques based on documented literature results that include simulation studies, lab and yard experiments, field implementation, candidate selection, operational requirements, technology enhancements, advantages, limitations, and challenges of each technique. The paper provides a comprehensive summary of different tunneling techniques focusing on the operational practices, tunneling mechanisms, tunneling depth, and recent advancements available in the market. The most effective applications of the tunneling techniques are in stimulating low permeability, depleted and thin reservoirs, layers close to water zones, and bypassing near wellbore formation damage. The efficiency of creating tunnels is affected by many factors such as reservoir properties, nozzle, and fluid types, etc. The tunnel shape and trajectory are affected by reservoir geological properties. The combination of the tunneling with other stimulation techniques can result in more effective treatments, which enhance the methods of current stimulation. Reservoir tunneling technologies can pave the way to improve hydrocarbon recovery and enable access to unstimulated formations.
A Review: Progress of Diverter Technology for Oil and Gas Production Applications in the Past Decade
Diversion technologies is becoming widely used as part of multistage fracturing operation and acid stimulation especially in carbonate formations completed with extended reach or multilateral wells. Further importance is gained during the development of unconventional resources where large number of stages are required with enhanced stimulated reservoir volume (SRV) per fracture. This is achieved by improving the fracture network and complexity using far field or deep diversion techniques. Diversion gained more value since it was an enabler for more efficient refracturing jobs since it can divert treatment from existing fractures. One of the main functions of diverters is to direct the stimulation fluid toward the desired treatment interval to increase the efficiency of productivity enhancement process. A diverter could be either mechanical or chemical. Mechanical diverters include packers, ball sealers, coil tubing, and particulate diverting agents such as benzoic acid flakes, rock salts, wax beads and fiber. Chemical diverter is mostly used as temporary barrier of fluid during treatments, and will get converted back afterwards by chemical means. Chemical diverter can be divided into two main types: polymer-based diverter and surfactant-based diverter. In the past decade, biodegradable diverter has been developed according to the concern of both environmental protection and less formation damage. Relative permeability modifier (RPM) can also be used as diverter in some cases. All the above diversion techniques will either divert the fluid in the wellbore or deep inside the formation based on the objective of the treatment and type of fluid used. This paper covers diverters in both injectors and producers with the applications of matrix acidizing, acid fracturing and hydraulic fracturing. In matrix acidizing, polymer-based acid gel is one of the most applied diverters. Adding N2/CO2 to form foamed acid, the treatment efficiency could be further enhanced with less formation damage. Viscoelastic surfactant (VES) improved acidizing was also applied in many cases. Fiber based acidizing fluid proposed to be effective in carbonate formation. Multi-stage acid frac jobs were done in 2011 in tight gas carbonate formations. A new trend of acid frac is to use CO2 energized fracturing fluid for tight, sour gas formations. Far-field fracturing mechanism was studied by means of solid particulate diverting agents. Eco-friendly and biodegradable diverters were applied for zonal isolation. Nanoparticles, as new generation of diverters, have been used for EOR as foaming agents since beginning of this decade, especially at HTHP conditions; nanoparticle stabilizers were applied in polymeric gel and VES system to enhance the stability for diversion fluid. To make the best performance of diverters, limitation on working conditions of each type of diverter would be identified, such as cost, temperature range, pH range, size distribution, and compatibility with fluid additives.
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