Pumping schedule gets acid past damaged zones and generates wormholes.

The barge was rigged up for maximum safety and efficiency in the stimulation operation.

Malaysia is a significant gas producer and liquefied natural gas exporter within the Asia-Pacific region. Many of the country’s gas fields are offshore with carbonate reservoirs. The exploitation of these resources involves drilling horizontal wells for maximizing reservoir contact and hydrocarbon drainage.

Many of these wells experience drilling mud damage. One of the challenges in stimulating long horizontal wells with openhole completion is the placement of stimulation fluids for effective zonal coverage and generating wormholes to pass the damaged zone. Placing gelled acid through coiled tubing (CT) has been the standard industry practice to clean up a well bore, but due to the low pumping rate, stimulation results have been limited.

Shell executed a stimulation campaign in 2006 to use the systems selected in long open-hole horizontal carbonate gas wells. Through the stimulation of seven wells, we achieved significant gain through progressively optimizing pumping procedures and observed indications of diversion compared to previous practices. Stimulation cost turned out to be less than that of using CT.

Introduction

In Malaysia, long horizontal wells are drilled where there is no compartmentalization or baffles in the carbonate reservoirs. In addition, the wells are completed with large, 7 5/8-in. tubing. The wells are cleaned using a clean-up package; stimulated with acid; then further cleaned up before being tied into the production facilities.

The previous stimulation technique applied to carbonate gas wells completed with long

horizontal open holes (with or without pre-drilled liner) was to run in CT to spot acid (straight or gelled), while pulling the CT from the toe to the heel with a maximum pumping rate of less than 2 bbl/minute (limited by the available CT unit). This limited pumping rate with CT (<2 bbl/min) was not sufficient to stimulate the wells, i.e., acid was unable to pass beyond the damaged region and generate wormholes that would penetrate into the formation.

Realizing that results from previous treatment with CT did not increase well productivity as expected, especially for wells with long horizontal sections, stimulation alternatives within the current business infrastructure were considered. In mid-2005, an invitation for technical proposals to effectively stimulate extended-reach horizontal carbonategas wells with openhole completion was sent to service companies in the region.

Objectives

Our intention was to look for a matrix stimulation (not fracture stimulation) proposal from a service vendor that contained both an acid system and a pumping procedure with relevant equipment and technique to pass the damage region and generate wormholes to reduce the skin and penetrate deeper into the formation. Such a stimulation operation was expected to significantly improve well productivity.

Challenges

Ideal results of acid matrix stimulation in carbonate reservoir should cover the whole

horizontal section with evenly distributed acid treatment and subsequently generate enough wormholes to penetrate into the formation to bypass near wellbore formation damage. If permeability and damage contrast is significant, the diverting technology would help in placing acid evenly throughout the open hole.

In order to improve the productivity of the new wells to be drilled in four gas fields, Shell decided to seek proposals from service companies to provide techniques and engineering methodology to achieve such a goal in a drilling campaign.

During the tendering process, Shell delineated several criteria, including:

  1. Evenly place acid to cover the whole horizontal open hole section (from toe to heel). A typical well configuration for those wells is illustrated in Figure 1.
  2. Generate wormholes in the formation to reduce/bypass skin.
  3. Divert and place acid to stimulate at least a large portion of the openhole section or a technique to achieve the same effect.
  4. Treat below fracture pressure (matrix stimulation only).
  5. Bring field-proven techniques and acid systems with success records in similar situations within and outside Shell.
  6. Offer an advantage over current practice (CT + gelled acid) in delivering better results.
  7. Provide a detailed list of equipment and tools needed and their specifications.
  8. Come with technical expertise to design and supervise the jobs on site.
  9. Provide health, safety, and environment (HSE) information about the technique and products. Assurance on HSE is one of the most critical screening criteria. The products provided should not cause damage to well accessories with minimum treatment by using corrosion inhibitors.
  10. Provide study and modeling results using proven software or model base on provided well data to show the advantage of the proposed techniques.
  11. Work in current field conditions.
  12. Be available in the designated location and time frame.
  13. Provide job evaluation (technical and operational) after completion of the stimulation job.

Several companies participated in the exercise. Mature technologies that have been applied within Shell and elsewhere globally were proposed as expected. They all had a proven track record of success. In terms of acid systems, they varied from self-diverting surfactant acid systems to in-situ crosslinked gelled-acid systems. From the engineering point of view, the key factor of success in pumping any of the proposed acid systems is to effectively divert the acid to cover the whole section of the well and have enough pressure to drive the acid into the formation to generate wormholes. To achieve such results, a high pumping rate is essential.

Bullheading was proposed by all participating companies that aimed to achieve maximum pumping rate. Available pumping capacity in the region, however, is limited. None of the participating companies could provide a pump rate higher than 8 bpm without Shell committing a volume of work. Simulation by Stim2001 (a Shell internal-acid stimulation-design program) indicated that with such a pumping rate limitation, the expected gain would be less than that with a high rate, but still higher than that obtained from previous practice with CT, whereas the difference between various self-diverting acid systems was not significant. Thus, an in-situ crosslinked gelled acid system was selected.

Two diversion systems were used during the matrix acidizing campaigns, each based on different mechanisms. The first system was based on in situ crosslinking that allowed for self-diversion of the acid. The in-situ crosslinked system is applicable at temperatures exceeding 250°F (121ºC). The second system utilized was based on particulate diversion technology. The particulate system is applicable at temperatures below 250°F.

Field implementation

An acid stimulation campaign was implemented in four gas fields. Seven wells were treated, two of which were vertical wells that were cemented and perforated.

As computerized mixing systems were not available in the region at the time of the campaign, a modified mixing-on-the-fly system was used to optimize acid mixing and utilize available barge space. The system is controlled manually by on site engineers. Before mobilizing the system to location, several yard tests were run to ensure the system was operable and to identify parts that wear out easily. Spare parts were prepared for a long stimulation campaign. Figure 2 displays the equipment layout in a tight space on an acid stimulation barge. It contains the mixing-on-the-fly system called process control acid module (PCAM).

Advantages

The PCAM ensured continuous mixing to eliminate the need for batch mixers, storage tanks and numerous connections and fittings. The mobilization logistics to and from location were simplified tremendously. The need for large supply vessels was also eliminated. Crane capacity and the number of lifts were reduced. Required deck

space to position equipment was also minimized. Using smaller supply vessels saved costs and allowed for faster mobilization and demobilization times. Smaller platforms and jackets could be accessed and smaller workboats used. Since less equipment space was needed, more area on deck could be used for storing chemicals, reducing the number of replenishment trips during big acid campaigns. By optimizing logistics, more wells could be acidized in locations that were formerly inaccessible. Workspace requirements were typically reduced by 30 to 40%.

The continuous mix/process controlled acidizing used only liquid additives. The additives and concentrated acid were stored in tote tanks and carboys that were manifolded together, and the liquids were fed directly either to metering tanks or to the PCAM. There was no cutting of sacks and no transferring of liquid chemicals from drums. This eliminated dust from unloading sacked chemicals, and spills, while transferring liquid additives from drums. Inhalation, physical contact and exposure to these chemicals were reduced. Acid systems were no longer stored in tanks on surface. Fuming acid, the possibility of leaking tanks and spills were dramatically reduced or eliminated.

In order to evaluate well productivity and the degree of formation damage, the wells were

cleaned up and flow-tested before implementing acid stimulation. This practice produced several insights.

•        Understand the well and reservoir before conducting stimulation. Pre-stimulation

productivity data (rate vs. tubing head pressure data) are very useful for post stimulation evaluation.

•        The wells are filled with reservoir gas. Due to the large wellbore volume, surface treatment pressure is reduced during a large part of the treatment period.

•        Rocks in the near wellbore region are saturated with gas. This permits best acid

placement and diversion.

  • Avoid placing a large volume of wellbore fluid (drilling fluid or completion brine) into the formation. This reduces additional formation damage by the wellbore fluid.

Pumping procedure

Bullheading acid into a well bore that is filled with gas requires a guideline in surface treatment pressure to avoid fracturing the formation while trying to pump at maximum possible rate. So, during the treatment design stage, we required that the surface pumping pressure prediction and maximum treatment pressure limitation be put into the design program. During pumping, this limitation should not be exceeded to prevent fracturing the formation. This guideline was strictly observed during the whole campaign.

At the beginning of the campaign, due to the limit of the pumping rate, acid was pumped at 8 bbl/minute and displaced with seawater at the same rate by using the same acid pump. It was realized that by displacing the acid at high rate up to 34 bbl/minute with a rig pump would enhance the diversion more effectively. This pumping schedule was used for most of the wells after treating the first two wells.

Flow back testing

After the acid was placed into the formation, a two-hour soaking period was allowed for the acid to fully react with formation rock and for the crosslinked gel to break. This was to allow for easy flow back of the spent acid. Then the wells were switched to flow back mode. Gas rate, liquid loading, tubing head pressure (THP) were measured with a production testing unit. In all the cases, the wells were flipped into gas flow condition immediately.

Results and best practices

In one of the stimulated gas fields, wells exceeded planned productivity by a large margin. Table 1 summarizes the gain of the seven wells in the acid stimulation campaign. Comparison of THP versus production rate curves (normalized for data security) of pre and post stimulation showed that by improving the pumping rate, more effective diversion was achieved, leading to better stimulation results. Bullhead pumping to place acid with diversion and using a rig pump to achieve high rates was more effective than the previous practice. The procedure was also more cost efficient than using CT.

From this high success rate acid-stimulation campaign, we have come up with the following best practices for future stimulation.

  • Bullhead pumping acid at high rate with diversion acid systems is more effective than previous treatments with CT.
  • Promising results can be observed as the pumping procedure improves.
  • Equipment limitation does not inhibit attaining optimizing pumping procedure.
  • Unit gain/cost is more economical than using CT.
  • Potential improvements can be found in future campaigns.
  • On site quality assurance/quality control is a key element to ensure a high success rate.
  • Cooperation between well engineering, operation, production technology, production chemistry, and well-testing disciplines is the key to success.