High-temperature reversible invert fluid enhances drilling and completion performance in carbonate reservoir.

A recently completed horizontal gas producer in a carbonate reservoir in the Gulf of Mexico presented numerous challenges for the drilling and completion phases. The primary objectives for this completion were to transect the target carbonate reservoir with a maximum horizontal well path and subsequently stimulate the entire lateral with an effective cleanup system while mitigating any formation damage. The challenges included: bottomhole temperatures that exceeded 300°F (148.7°C), a relatively low-permeability reservoir that exhibited a wide range of isolated but relatively large pore openings, cleanup of a relatively long openhole lateral, "acid" gases that included H2S and CO2, and the optimization and design of the fluid and tool systems while mitigating costs.
To meet these challenges, planning and laboratory testing were performed to optimize a reversible oil-based reservoir drill-in fluid (RDF) and a reversible solids-free system for the horizontal displacement. Of key concern were prevention of potential incompatibility, effective cleanup of the residual filter cake, and maintenance of inherent wettability. It was decided that the RDF must also meet or exceed local environmental regulations, minimize invasion and circumvent excessive fluid losses, inhibit any reactive shales and maintain the integrity of the RDF system with respect to the high-temperature environment.
The petroleum industry defines high pressure/high temperature (HP/HT) generally as greater than 10,000 psi and 300°F. High static temperatures can affect mud properties and LWD and MWD tool life. Oil-based drilling muds (OBMs) are normally used when bottomhole temperatures exceed 300°F, partly because of their stability and predictable fluid properties that usually require minimum treatment and thus, low relative additive costs.
OBMs are normally reusable, drill gauge hole, exhibit lower fluid loss resulting in a thinner filtercake, tolerate solids, saltwater, CO2, and H2S contamination and provide lubricity. However, OBMs are susceptible to certain problems. Among these are natural gas solution and reduced rate of penetration from increased solids. The latter is generally the result of an OBM tolerance to higher concentrations of low-gravity solids. The result is the OBM is stable; however, the rate of penetration decreases. Another potential problem is the presence of water in the filtrate, which, is usually the result of thermal degradation of the inherent emulsifier package. Left unchecked, this phenomenon can lead to wettability changes and full-scale breakdown of the OBM. An OBM can also have a cost disadvantage because of the potential for lost circulation.
Water-based muds (WBMs) are not usually the choice for high-temperature wells because of thermally sensitive components. For example, WBMs use organic polymers that, if not treated for thermal degradation, can lead to hydrolysis. In addition, high-temperature gelation of WBM is due, in part, to low-gravity solids and can lead to an adverse increase in shear strength. The free water associated with WBM has a tendency to "cook," making the mud unstable and ineffective.
Rheological control is also a difficult challenge in a high-temperature environment. Formates are usually the base brine of choice as a solution to the thermal integrity issues, but economics may preempt this fluid as a choice. WBMs are cost effective and environmentally friendly, which are desirable qualities.
In April of 2003 the authors initiated a comparative testing program that included confirming the integrity, cleanup, inhibition, and emulsion tendency of a reversible invert OBM, conventional OBM, and a WBM for drilling and completing a 2,400-ft (732-m) lateral in the Mobile, Ala., area of the Gulf of Mexico. A horizontal well had been selected to accelerate reserve recovery. The horizontal would be drilled laterally in an attempt to maximize the intersection of good quality reservoir rock. The rock properties appeared to vary on a foot-by-foot basis. A horizontal well was also expected to remain above any possible gas/water contacts. Of key concern was the ability of the selected cleanup system to effectively remove the subsequent residual filtercake and any potential near wellbore damage formed from any of the RDF systems. There was a concern that oil based mud emulsifiers might block the minimal porosity/permeability. In addition, there was a concern from a drilling perspective that WBM would not permit the directional control required to drill the interval.
Reversible OBM explained
A reversible invert emulsion drilling fluid is readily converted from a water-in-oil emulsion to an oil-in-water emulsion and potentially back to a water-in-oil emulsion using an acid-base chemical switch (Figure 1). A novel surfactant package forms an invert emulsion in the presence of lime and a regular emulsion in the presence of acid. Protonation and deprotonation of the surfactant can be used to control the emulsion phase. However, the reversibility function at or greater than 300°F had not previously been tested in the field. The use of HCl/acetic acid readily and quickly changes the residual filter cake to a water-wet state and promotes disaggregation of the entrained solids. The bridging solids entrained in the residual filter cake are calcium carbonate; once water wet they are readily consumed by the acid. However, more detailed testing was required to fully test the functionality of the system for the anticipated reservoir conditions, primarily a bottomhole static temperature (BHST) of 295°F (146°C). However, the bottomhole circulating temperature (BHCT) was measured while drilling and ranged from approximately 260°F to 299°F (126.5°C to 148°C). This suggested that the BHST was much higher. The benefits for using a reversible RDF system in this completion include the ability to water-wet solids thereby mitigating plugging/sludging of the completion tools and the ability to use chemicals for a water-based cleanup.
Project location
The proposed well was planned in the Mobile field, located off the coast of Alabama and Mississippi in the Gulf of Mexico, almost due south of Pascagoula. Several pipeline gathering systems currently exist in this area. The proposed target was the Lower James Limestone. As the well was planned as a horizontal gas producer, the well path would traverse the Middle James Limestone. It was desired to set the 7-in. casing across the partially depleted Middle James before drilling the horizontal. To date, wells completed in Mobile blocks 991 and 992 have produced approximately 32.81 Bcf.
Completion background
The horizontal was planned to intersect the top of the Upper James at a total vertical depth of approximately 15,200 ft (4,636 m) at 90° and set 7-in. casing. After cementing and drilling out, a 6-in., 2,400-ft (732 m) lateral would be drilled to a total measured depth of approximately 18,800 ft (5,734 m). Subsequently, the residual RDF would be displaced to a solids-free system and a ported-sub would be used to complete the lateral. The perforations in the ported sub would be optimized by aligning with the most optimum reservoir rock. Next, a flow test would be conducted without cleanup. Subsequently, the lateral would be acidized with a 15% HCl/10% acetic based system. It was anticipated that initial production rates would approach 8 MMcfg/d to 10 MMcfg/d and 150 bwp/d to 200 bwp/d. The James Limestone carbonate has an estimated permeability that ranged up to a maximum of 20 mD. Based on a pore pressure of approximately 8.5 lb/gal and field experience, a 9.2-lb/gal RDF system was required to drill this formation. It was also anticipated that trip and connection gases would contain CO2 at about 5% and H2S with estimated concentrations of just under 30 ppm.
Laboratory testing
A laboratory testing program was undertaken to identify an optimum RDF. Three RDF systems were proposed: a reversible OBM-RDF, a conventional OBM-RDF, and a WBM-RDF. Of key concern was the ability to maintain rheological properties throughout the drilling of the lateral and subsequent completion. The laboratory data would be used to establish rheological and property targets and to develop a field maintenance schedule. In this manner, the RDF targets could be communicated to the field personnel for the purpose of establishing protocol and recommendations for common field problems that would not compromise the selected RDF systems integrity or potential to mitigate formation damage. It was also established that the laboratory data would be compared to the field data in order to document the differences to establish not only the validity of the selected RDF performance but the need for improving any properties that did not meet the proposed targets. As such, the laboratory analyses would include petrophysical, compatibility, rheological, fluid loss, thermal integrity, and cleanup.
Primary concerns for the RDF system included the following:
• Maintain the integrity of the RDF system;
• Prevent potential emulsions;
• Mitigate potential wettability changes;
• Minimize invasion and circumvent excessive losses;
• Effective clean up of the residual filter-cake; and
• Meet or exceed environmental objectives.
Drilling results
In addition to standard drilling mud parameters that are generated and reported every 12 hours, selected key parameters were monitored. Contingencies were established in the form of a system maintenance schedule (prepared in advance) to address potential reversible-RDF incompatibilities, contamination from reservoir, drilling or completion fluids, and thermal stability. The primary objective was to maintain the reversibility of the reversible-RDF system at all times. A reversibility target of 20 ml was established and contingencies included treating with primary emulsifier and reducing low-gravity solids (LGS). Another key objective was to maintain stability of the reversible-RDF system in lieu of the elevated bottomhole temperature (BHT). Two key parameters, the excess lime and ES targets were established as greater than 1.5 lb/bbl and 300 volts, respectively.
Reversibility fluctuated between 15 mL and 65 mL (Figure 2). The elevated values are due, in part, to slugs and the reversible RDF system's exposure to BHST during trips. Numerous trips were required for bit and bottomhole assembly (BHA) changes. This was largely related to the elevated BHT. The highest values were recorded when a test was performed after a sample was caught from bottoms-up. The most stable values occurred while drilling the last 300 ft (91.5 m) because no trips were performed. Field permeability plugging apparatus (PPA) values are shown in Figure 3. These tests were run on an aloxite disk using 1,000-psi differential pressure at 300°F. The 30-minute totals, approximately 6 mL to 12 mL, agree well with laboratory PPA fluid loss simulating the same wellbore conditions.
Displacement results
The first displacement used a reversible RDF SF system. Approximately 125 bbl of 9.3-lb/gal reversible RDF SF were used. This covered the lateral and approximately 300 ft of the 7-in. casing. The displacement began using a 3-bbl/min rate. When the leading edge of the reversible RDF SF pill reached the toe of the lateral, the rate was decreased to 1 bbl/min. The displacement continued at this rate until the reversible RDF SF and a slug were theoretically spotted in place. No fluid losses were observed during this displacement. The second displacement utilized a five-spacer train. The calculated casing volume for this displacement was approximately 754 bbl. Nephelometric turbidity units (NTUs) were monitored and after pumping approximately 0.94 hole volumes, the clarity dropped to 17 NTUs.
Completion assembly and production pesults
The cased interval was subsequently displaced to 9.5-lb/gal NaBr and dual external casing packers (ECPs) were run to isolate bottom of hole at approximately 18,409 ft (5,614.7 m). Next, a 3½-in. ported-sub assembly was run that consisted of 10 separate subs. Finally, a packer was run and set at approximately 16,168 ft (4,931.2 m). The drillstring was displaced with nitrogen to 10,000 ft (3,050 m) and the well did not flow. The drillstring was then displaced to approximately 16,000 ft (4,880 m) with nitrogen and tested for 14 hours. Initial test data showed a production rate of 2.8 MMcf/d with 1,626 psi flowing tubing pressure (FTP). The final rate exhibited approximately 5.5 MMcf/d
with 1,151 psi FTP. The solids-free
pill was subsequently produced back and remained as an invert emulsion during the entire completion phase. The lateral was subsequently stimulated with approximately 60,000 gal of acid using nine diverter stages followed by completion brine. A rate of 30 bbl/min was attained. The well was opened up to the test unit and tested for 44.5 hour. The well exhibited a final rate of 18.9 MMcf/d with 1,648 psi FTP on a 64/64-in. choke. This production included approximately 32-bbl/hr water.
Conclusions
1. The ported-sub method worked well with the acid stimulation;
2. The reversible oil-based RDF cleaned up well; in addition, the solids-free reversible fluid exhibited no detrimental residuals;
3. The reversible oil-based RDF exhibited good rheological properties while drilling, however, extended periods out-of-the-hole resulted in deteriorating reversibility of the fluid system, whereby additional treatment was necessary to maintain this property;
4. Losses were noted while running the ECPs and were most likely the result of disturbing the filter cake while running in hole;
5. This was the first application of a reversible invert-emulsion system in a high temperature (>300°F) horizontal well drilled and completed in a carbonate reservoir in the Gulf of Mexico;
6. This well exhibited sustained initial production of approximately 15.88 MMcfg/d and 216 bw/d on a 32/64-in. choke as this rate was higher than anticipated;
7. Based on initial production and completion data, the reversible RDF solids-free system exhibited good stability using a 13.7-lb/gal NaBr/CaBr2 internal phase. Initial produced fluids were emulsion-free and exhibited good phase separation; and
8. The laboratory optimization testing for the 9.2- lb/gal reversible RDF solids-free system compared well with field application.

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