Nonaqueous, Nondamaging Fluid Implemented Offshore Abu Dhabi
This paper covers the 7-year history of drilling-fluids application in a reservoir drilling campaign offshore Abu Dhabi, from the early use of a solids-free, brine-/water-based mud to the recent application of nondamaging, nonaqueous fluids (NAFs) with micronized acid-soluble ilmenite.
This paper covers the 7-year history of drilling-fluids application in a reservoir drilling campaign offshore Abu Dhabi, from the early use of a solids-free, brine-/water-based mud to the recent application of nondamaging, nonaqueous fluids (NAFs) with micronized acid-soluble ilmenite. The complete paper provides details about the integration of filter-cake breakers with various types of drilling fluids, from dormant drilling-fluid additives to delayed, pH- and temperature-activated breakers. The paper describes the steps taken to achieve these performance goals, including changes to the oil/water ratio (OWR), internal phase composition [heavier calcium briomide (CaBr2) instead of calcium chloride (CaCl2)], and a micronized, acid-soluble ilmenite as a weighting agent.
A key objective for the offshore Abu Dhabi project was to drill longer horizontal 8½-in. intervals while retaining zero-skin completions. This necessitated reducing the torque and drag and coefficients of friction (COF). Achieving these design criteria required developing a nondamaging, nonaqueous (NAF) drilling fluid complemented with a filter-cake-removal breaker system. Emphasis was placed on the design of an NAF that combined the desired properties of low rheological profile for equivalent-circulating-density (ECD) management, low COF, and nondamaging water-based-mud (WBM) formulation.
Four different generations of breakers for water-based reservoir drill-in fluids (RDF) were developed and used before moving to nondamaging NAF. The NAF went through three iterations in addition to fine-tuning of the breaker package.
Data related to well information, reservoir rock type, and completion type were gathered and analyzed. Fluid interaction and other studies were performed to determine suitable fluid type and formulation. Other factors taken into consideration included offset well data, drilling requirements, environmental considerations, logging requirements, and likely mud-damaging mechanisms. Extensive laboratory tests were conducted, some of which included compatibility of various fluids, return permeability, changes to the OWR, internal phase composition, and a micronized, acid-soluble ilmenite as a weighting agent. The breaker systems saw the same extent of refinement, from enzymes to delayed organic acid precursors and chelating agents, to evaluate the removal of the fluid filter cake by the breaker. Fluid formulations were evaluated and optimized.
More than 80 extended-reach-drilling wells have been drilled using both water-based and nonaqueous RDFs, each with specially formulated breakers. These wells yielded learnings that contributed to the current design and formulations.
Friction factors (FFs) obtained using RDF NAF proved to be much lower than those with RDF WBM. The lower FF enabled wells with longer horizontal sections to be drilled successfully and at significantly higher rates of penetration. Using micronized, acid-soluble ilmenite lowered ECD as compared with sized calcium carbonate. Additionally, the evolution of breaker formulations allowed for longer breakthrough time, which allowed for better coverage of the lateral, better removal of the filter cake, and, ultimately, enhanced production through improved inflow profiles. The end result of the continuous improvement in reservoir drilling fluid was an NAF that combined the desired properties of low rheological profile for ECD management and low COF, and was nondamaging.
First Phase: Nondamaging Water-Based Drilling Fluids
The water-based RDF was designed for minimum formation damage and maximum in-situ filter-cake removal. The original plan was to perform acid stimulation with coiled tubing, with the main concern being the possibility of not reaching the total depth of the 8½-in. section because of drag limitations. To address this concern, the filter cake was designed with internal breakers for self-dissolution, given that acid may not be available for either cake destruction or formation stimulation.
The complete paper details the fluid’s properties and additives, and discusses the design criteria for the filter-cake-removal solution to maintain lubricity and viscosity while destroying the viscosity and dissolving the filter cake without an acidization step. The breakdown solution was a 10.5- to 11.5-ppg sodium bromide-based blend.
Second Phase: Nondamaging NAF
To maintain filter-cake removal ability with the breaker system, the fluid had to be barite-free. In the absence of barite, the remaining options to weight up an NAF were as follows:
- Use calcium carbonate as a weighting agent; however, barite has a specific gravity of 4.2 compared with 2.6 for calcium carbonate, resulting in much higher solids loading.
- Change the OWR from 80:20 to 65:35 to take advantage of the higher-density CaCl2.
Both of these methods would dramatically increase the plastic viscosity (PV) and subsequently increase ECD. After one trial with the CaCl2 internal-phase system, the system was redesigned to lower the PV. Using a 14-ppg density CaBr2 brine as an internal phase instead of the 10.4-ppg density CaCl2 enabled decreasing both the OWR and the overall loading of solids, affecting the PV directly and, subsequently, the ECD.
Another important step was to confirm the efficiency of the breaker formulation. Following laboratory testing and field evaluation, the proposed formic acid was replaced by a formic acid precursor to delay the breaker time. Also, as for the nondamaging WBM RDF, hydroxyethylcellulose was used in this blend to slow the reaction rate. It does this primarily by providing a viscous matrix through which the breakers must penetrate/diffuse to access the filter cake.
A challenge still existed with the need to design a nondamaging NAF to exhibit very low PV for reduced ECD to allow drilling 8½-in. intervals of more than 15,000 ft while retaining zero-skin completions. To solve this challenge, while the CaBr2 internal phase was retained, a new, acid-soluble, higher-specific-density weighting agent was required. The solution was to use micronized ilmenite as a weighting agent. Ilmenite has a specific gravity of 4.6 (vs. 2.6 for CaCO3) and is acid-soluble in both formic acid and HCl.
The authors devote several pages to a discussion of ilmenite, which was recognized as a potential drilling-fluid-weight material more than 40 years ago, but failed as a direct substitute for API barite, not because it offered poorer performance, but because it did not offer better value. The paper explains that a new and optimized micronized ilmenite offers significantly better performance and value. It is a dry ilmenite, which has been micronized to a particle size distribution with a D50 of approximately 5 µm and a D90 of approximately 12 µm. The authors present charts, photos, tables, and lists highlighting the performance and health, safety, and environmental advantages of the new micronized ilmenite.
In Abu Dhabi, a 10.5-ppg micronized ilmenite-based NAF had a PV of 16 cp, compared with 27 cp for a CaCO3-based NAF (both with CaBr2 internal phase). Because the lower PV leads to lower ECD, drilling and completion of longer horizontal sections was enabled. The micronized ilmenite-based NAF has a filter cake removable in the formic acid breaker, so no changes were required for the breakdown solution formulation.
Field Application and Results
The authors write that there have been tremendous improvements throughout the execution of the offshore Abu Dhabi drilling program. The mud formulation has been improved continuously to yield higher stability and augment drilling efficiency.
The first order of business was to reduce COF to allow for drilling and completion of longer laterals. Switching to an NAF allowed drilling with as low as 0.1 COF with very little breakover recorded. This enabled drilling record laterals in the UAE and running record-length liners in the pay zone (Fig. 1). ECD has been as low as 0.8 ppg, enabling longer laterals without exceeding fracture gradient, and increasing the flow rate for better hole cleaning (Fig. 2).
To evaluate the efficacy of the breaker package with the field fluids, flowback tests were performed with the actual field drilling fluids using 10-µm ceramic disks. Results indicated that the breaker solution was very efficient, rendering a return flow as high as 92%.
- A series of both water-based and nondamaging drill-in NAFs was formulated, optimized, and used offshore Abu Dhabi.
- One of the most important developments during this project was the design and implementation of a nondamaging NAF.
- A breaker completion fluid was implemented to augment filter-cake dissolution and eliminate the need for a post-completion acid stimulation.
- Production from all the wells drilled with this drill-in/completion fluid package exceeded expectations and maintained the rates over time.
- The breaker package has been optimized to work for both water-based and nonaqueous formulations, on the basis of formic acid precursors, calcium carbonate chelant, mutual solvents, and surfactants.
This article, written by JPT Technology Editor Judy Feder, contains highlights of paper SPE 192735, “The Evolution of Nondamaging Fluid Design and Implementation Offshore Abu Dhabi,” by Catalin Ivan, SPE, ExxonMobil; Yousif Saleh Al Katheeri, Melanie Reichle, Khalid Akyabi, James Thomas Ryan, Sheldon Peter Anthony Seales, SPE, and Sigit Kustanto, SPE, ADNOC; Laurie Hayden, M-I SWACO; and Christoper Steele, SPE, ELKEM, prepared for the 2018 Abu Dhabi International Petroleum Exhibition and Conference, 12–15 November, Abu Dhabi. The paper has not been peer reviewed.