A well planned and executed well stimulation is one of the primary methods to enhance production from both oil and gas reservoirs and increase injectivity to injection and disposal wells. Some key components for optimal stimulation treatments are the chemicals or treatment fluids themselves, the length and location of interval(s) that require stimulation and also the deployment or diversion method to be used. With the economics of some treatment fluids in the industry today more of an emphasis is being put on accurate placement of these fluids. This is highlighted more in marginal fields that may have a low reservoir pressure. When stimulating these wells with coiled tubing (CT) as the deployment method, control of the treatment fluid is paramount. In a high percentage of these low reservoir pressure wells a high overbalance in favor of the CT may exist, leading to fluid fall out of the coil at an uncontrolled rate.

The Baker Oil Tools through-tubing Inflatable Straddle Acidizing Packer (ISAP) is designed to allow the operator to precisely inject acid or chemical treatments into targeted sections of the formation. This feature eliminates the use of other less effective methods used in the past for selective placement of treating fluids, such as ball sealers or chemical diverters. The ISAP is designed to run primarily on CT, thus the tool operates within the safe tension, set down and pressure limits of the coil. The tool requires only a slight amount of tension, and no set down weight for operation. This makes it ideal for horizontal or highly deviated wellbore applications. Using CT conveyance allows the ISAP to be run in a "live well" condition, eliminating the need for killing the well prior to running the tool. This prevents possible formation damage due to heavy weight kill fluid in the well. The ISAP is designed to be resettable, thus a complete zone or interval in the wellbore may be treated with a single trip in the well. The inflatable elements have also been designed, and field proven, to be set in perforations, so long, continuous intervals may be treated without having to have a blank casing packer seat in which to set the elements. The spacing between elements is adjustable, depending on customer preference, with the simple addition of threaded spacers between the top and bottom packer sections.

The tool is of modular design allowing it to be made up to the bottom of the CT as it is being pulled up into the CT lubricator.

Operations of the tool are controlled using a three ball system design. The first and smallest ball (0.38-in. or 0.50-in. diameter, depending on toolstring diameter) is circulated to the tool through the CT to initiate the setting operation, once the tool is at setting depth. The next larger ball (0.63-in. diameter) is used to open a circulation port above the tool once the treatment operation has been completed, which allows a circulation path through the CT while tripping out of the hole. The largest ball used [0.75-in. or 0.875-in. outside diameter (OD), depending on toolstring diameter] is only used should the ISAP be required to be disconnected from the CT in an emergency situation. Only the smallest ball, however, is required to initiate the setting and operational sequence of the tool, thus a ball is not circulated to the tool for each setting.

After the setting ball has been pumped to the tool, the ISAP has two operational positions that are controlled by application or release of tension on the tool. The first position is the spotting/equalize position. The tool is in this position whenever tubing forces on the tool are neutral or compression (set-down). The second position, the injection position, occurs whenever 2,500 lb to 3,000 lb overpull (tension) is applied to the bottomhole assembly (BHA). The ISAP does not rely on CT set down weight for position manipulation, as this force was deemed unreliable in deep or horizontal wellbore applications.
Fluid control is also provided, both through the CT for inflation and treatment, and back through the CT in the event of a well control incident. An injection control valve (ICV) is used to maintain treatment fluids in the CT as the elements are deflated and the toolstring is moved to a new setting position. A pressure compensating back pressure valve has also been designed to run with the ISAP. This serves the traditional function of a back pressure valve in restricting upward fluid or gas flow up the CT, but also allows bleeding off any applied pressure below the flapper valves for proper operation of the ICV, which is solely pressure activated.

Case History #1

Project Objective.The objective of this intervention was to increase the injectivity rate in this water injection well from 4,100 b/d to around 18,800 b/d while also increasing the injectivity index and reducing the skin damage. This was to be achieved by performing a mud acid treatment across a 2,200-ft (671-m) horizontal section of 4.50-in., 12.6 lb/ft screens through a minimum restriction of 3.50 in.

Project Summary. To achieve the objective, a 2.50-in. OD ISAP system was selected to run on the 1.75-in. OD CT. Prior to installation of the toolstring and to ensure the internal cleanliness of the CT a 10 bbl pill of 10% hydrochloric acid was pumped through it. The BHA was made up in sections with a spacer length of 65 ft (19.8 m) being selected to run between the dual packing elements to allow for an effective cleanout and also to reduce the required amount of resets of the tool. The combination of treatment fluids pumped included ammonium chloride brine, butyl glycol acetate, 10% hydrochloric acid, 9:1 mud acid and 4% xylene. A flow regime of around 43 to 50 gal per minute was maintained throughout the stimulation with the surface pressure fluctuating between 3,800 psi and 4,600 psi.

Project Results. In total, four runs were performed with the ISAP system culminating in 33 sets of the tool with a combined total of 205,000 gal of fluid successfully pumped across the screens and into the formation. The end results of the stimulation achieved a far greater success than anticipated at the outset of the operation with an increase in injectivity rate from the 4,100 b/d to a new rate of 26,890 b/d while subsequently increasing the injectivity index by over 500% and successfully reducing the skin damage from 4.6 to 1.08.

Case History #2

Project Objective. From the outset of this application, the objective was to perform a mud acid treatment across 820 ft (250 m) of 5.50-in. Excluder screen through a 2.75-in. model "R" nipple in order to enhance the current water injection rate of 19,530 b/d to a higher rate of 31,450 b/d.

Project Summary. With a previous treatment across the entire 1,840-ft (561-m) length of screens with a standard washpipe assembly failing to enhance the injection rate and injectivity index, a new intervention method was sought to allow for a more precise placement of the acid treatment. With this in mind the ISAP system was chosen. Prior to running the ISAP, a production logging tool (PLT) was run to confirm the sections of screens to be treated. After analyzing the PLT data, it was determined that only an 820-ft (250-m) section would require stimulation. To guarantee an accurate CT set depth, a dedicated correlation run was performed by locating the 2.75-in. nipple with a nipple profile locator.

The ISAP assembly, dressed with 2.50-in. OD acid service inflatable elements to maximize fluids compatibility was picked up and deployed into the well. Upon achieving the first setting depth a 0.50-in. setting ball was pumped initiating the first set of the tool. Throughout the entire treatment the CT pump rate ranged from 21 gpm to 42 gpm at a maximum surface pressure of 3,800 psi.

Project Results. Nearly 80,000 gal. of fluids including ammonium chloride brine, 10% hydrochloric acid and 13.5:1.5 mud acid were pumped into seven targeted intervals. By employing a flow diversion system designed to target specific zones within the well, the injectivity rate was increased by 36,541 b/d to a total of 56,000 b/d. With this the injectivity index increased 185% from the conventional washpipe treatment that was performed previously and also reduced the skin damage from 4.4 to 1.