Advances in Dendritic Acidizing Reveal Light at the End of the Tunnel

Fig. 1—Section of an open horizontal well with acid tunnels and dendritic wormhole structure.

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Dendritic acidizing (DA) has a long history in the oil industry, going back at least to the 1980s. Significant technical advances include single-trip multitunnels using only acid-pumping equipment and a coiled-tubing (CT) unit, simultaneous tunnel creation, and improved parameter modeling and optimization. This paper provides an update on recent advances for DA methods, also known as acid tunneling, presenting a comprehensive review of published information for three different tunneling methods.


A well with multibranching major tunnels and a superimposed structure of wormholes emanating from the main tunnels has been called a dendritic well. This paper refers to methods that include the use of acids to construct such a well as DA. DA methods can be thought of as a hybrid between multilateral wells with many short branches with low skin factors and matrix acid jobs with very long primary wormholes and many branching wormholes from the primary wormholes. A conceptual example of a horizontal well section with formation tunneling and DA with branching wormholes adjacent to the primary tunnels is shown in Fig. 1 above.

Many different methods are used to form small-diameter lateral branches from a parent wellbore. The methods are differentiated by characteristics such as requirements for workover rigs, CT rigs, and deflector shoes for tunnel initiation; acid use in the process; and primary tunnel creation method.  

Method A

Method A uses a proprietary small-­diameter CT unit to deploy a jet nozzle on a flexible, retractable hydraulic hose. The method uses mechanical energy to blast the formation during tunnel creation and, in some cases, also uses acid either while jetting or while retrieving the jetting nozzle to chemically assist with tunnel creation and reduce the skin factor adjacent to the formation tunnels.

Method A can be applied in either openhole (OH) or cased-hole completions. In cased-hole applications, a deflection shoe must be run on a work string using a conventional workover rig, and then a hole must be cut in the casing with a separate milling tool. After the milling tool is retrieved by the proprietary CT unit, the jetting nozzle begins jetting a tunnel perpendicular to the well. Although the milling step is not required for OH applications, the deflection shoe must still be used. The jetting nozzle is deployed on a flexible tool that is kept perpendicular to the well by tension on the hose. The tension is also what pulls the hose into the tunnel, so Method A is a pull-the-nozzle method. Hose tension is created because there are rearward-discharging jets with greater discharge area than the erosional jet discharging toward the tunnel tip.

Method A tunnels are approximately 1 in. in diameter and may be up to 330 ft long. All the tools are fully retractable. Process limitations for Method A include minimum hole size and maximum depth. Currently, depths are equipment-limited to formations approximately 13,000 ft deep. The depth limit is controlled by the proprietary CT units currently available; however, units with greater depth capability could be purpose built to suit a specific project. The deflection mandrels cannot be deployed currently through anything smaller than 4.5-in. tubing because a 2.875-in. workover string must be run inside the existing equipment. The 2.875-in. work string carries, orients, and anchors the deflection mandrel. This is a significant disadvantage because both a rig for the work-string manipulation and a CT unit for the tunneling are needed and the CT gooseneck, injector, and blowout-preventer stack must be moved before the tubing can be manipulated by the rig. Also, only one tunnel may be created at a time, and Method A requires a manipulation of the deflection mandrel on the work string for each tunnel created.

Method A surveillance opportunities include job logs for rate and volumes pumped, tool depth, and line tension; pre- and post-job production logging tools (PLTs); and the use of different liquid tracers to mark the tunnels created at different depths.

Method B

Initially field-deployed in 2005, Method B uses acid to dissolve the formation rapidly and perhaps receives some benefit from hydraulic energy. A knuckle-joint deflection tool that is run simultaneously with the CT-deployed jetting nozzle makes the process unique. Method B uses retractable tools deployed on CT. The typically 10- to 23-ft-long bottomhole assembly (BHA) enters the tunnel and is pushed forward by the CT as the tunnel lengthens.

Method B is unique also because it is the only method considered that applies to OH completions solely.

Method B is a push-the-nozzle method. A tunnel of known length is formed; however, the path is not perpendicular to the wellbore and may be less effective in increasing the drainage volume of the well than Method A. Method B offers an optional memory inclinometer on the BHA to record tool position and orientation during the job; however, the inclinometer tool had not been used as of the writing of this paper.

Only a CT unit and the acidizing equipment are required for use of Method B. Method B only creates one tunnel at a time. Tunnel length is limited by the same mechanical forces that push the CT forward or recover it from any hole, and these effects can be modeled to help plan the jobs.

The BHA for Method B is 2.125 in. in diameter. Although the acid-jetting nozzle assemblies come in 2.25-, 2.5-, 2.75-, and 3-in. outside diameters, no obvious advantages can be seen to using a jetting nozzle assembly greater than the smallest available size. Modeling work verified that using smaller nozzle assemblies makes longer tunnels for the same volume of acid.

Very long tunnels, greater than 330 ft, are theoretically possible, although practical limits are created by friction and buckling. The process is also limited by the same considerations that limit the depth that can be reached by any CT job—namely, the ability to overcome friction without exceeding the CT tensile limit and the injector head pulling force limit when pulling the CT out of the hole.

In addition to memory-inclinometer and pre- and post-job caliper data, other surveillance data may also be useful. Just as in Method A, pre- and post- job PLTs may be useful, as well as applying different liquid tracers at different tunnel initiation depths. Memory or surface readout gyro data may be useful to measure tunnel paths.

Method C

First field-tested in 2014, proprietary Method C uses a completion-liner-­deployed system and nonretractable acid nozzles called needles to create multiple tunnels simultaneously. Liner subs containing up to four nonretractable needles are deployed between casing liner joints. Additional liner joints or pup joints may be used to create additional space between the acid needle subs. Hydraulic forces from the fluid flowing through the small needles overcome the jetting forces at the end of the nozzles to push the needles forward as the acid dissolves the formation to create a tunnel. Like Method B, Method C also is a push-the-nozzle method. The needles are small, and, in this case, the acid tunnels grow primarily because of chemical dissolution.

The distinguishing advantage of Method C is that many tunnels may be created simultaneously. The greatest number of acid needle subs that had been activated simultaneously as of November 2017 was 40, with four acid needles per acid ­needle sub, meaning 160 tunnels were created simultaneously across at least 40 liner joints.

A workover rig is needed for Method C, and the liner joints and needle subs must be installed in an OH interval. This can be performed during the initial completion of a well or during a workover, but a rig will be required to run the liner and liner subs in either case. The needle lengths, and consequently the acid tunnel lengths, are limited by the length of one liner joint, which usually is approximately 30 to 35 ft. Also, a maximum of four tunnels can be created for each liner joint, and the tunnel initiation points are at 90° relative phasing to the adjacent tunnels. Currently, no method exists to control rotational orientation of the liner subs or absolute phasing of the tunnels.

Surveillance opportunities for this method include prejob OH performance data and prejob PLT data. After the completion, the only inflow points into the liner will be through the needle ports, so, for any PLT, only measuring the flow rate inside the liner at a single depth per liner joint is necessary. No system exists to record the deployment status of the ­needles during the cleanout run; however, this could be a future surveillance opportunity to help optimize the method.

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper SPE 189945, “Dendritic Acidizing Update: The Light at the End of the Tunnel,” by C. Dean Wehunt, SPE, Stefan K.K. Lattimer, SPE, and Darren R. McDuff, SPE, Chevron, prepared for the 2018 SPE/ICoTA Coiled Tubing and Well Intervention Conference and Exhibition, The Woodlands, Texas, USA, 27–28 March. The paper has not been peer reviewed.

Advances in Dendritic Acidizing Reveal Light at the End of the Tunnel

01 June 2018

Volume: 70 | Issue: 6


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