By Chris West, Jim Hunt, Kevin Bowen, Chevron Bangladesh; Gary Cole, Greg McEwen, M-I SWACO
A combination of demanding well objectives in a remote and environmentally sensitive area prompted Chevron Bangladesh to achieve their drilling objectives by using an all-encompassing, fully integrated drilling fluid and drilling waste treatment plan for the Moulavi Bazaar (MB) and Bibiyana (BY) field development programs. This approach matched synthetic-base drilling fluid technology with a proven land-farming bioremediation treatment technology, which allowed the successful remediation of drilled cuttings within the limited time frame dictated by the local monsoon weather window.
The primary objective was to successfully reduce the total petroleum hydrocarbon (TPH) of the produced drilled cuttings to a minimum of 1% during the dry season. Secondary objectives focused on on-site management and deposition of remediated cuttings, as well as the use of a local labor force to assist with remediation activities.
This article reviews the planning and implementation of processes, quality assurance aspects and successful closure of the two bioremediation projects undertaken to date in Bangladesh.
The scope of the proposed field development wells to be drilled in the MB and BY north and south pad locations was as challenging as anything previously undertaken in Bangladesh. Extended-reach drilling (ERD) techniques are, by definition, already ambitious. Combined with local unpredictable tectonics, planned aggressive build rates, long tangential sail angles and strictly defined reservoir entry zones, drilling fluid selection became a critical factor in the project’s success.
The improved performance levels associated with oil- or synthetic-based fluids over water-based systems are well documented. However, the potential for increased environmental impact from an oil-based system is considerably higher as well. A performance-proven synthetic oil-based drilling fluid was selected for its ability to achieve all well trajectories and targets and comply with the operator’s environmental impact objectives.
The disposal of oil-based cuttings from land-based operations is a significant issue, especially in environmentally sensitive and protected areas, which in this case was the Lawanchara Forest, a dedicated nature reserve. The potential impact on soil and ground water can be significant if the waste streams generated are not treated in a manner that would permit biodegradation within the limited climatic weather window. As a result, drilling fluid components and overall fluid engineering design had to be optimised to meet degradation criteria without compromising overall performance.
Cuttings from land-based drilling can be treated by several methods. Solidification, thermal treatment and composting were identified as possibilities, but ultimately land-farming was selected, given its relative ease of control, historical success and the suitability of the chosen synthetic-base fluid fro bioremediation. The major impacts of this project were oil and grease (TPH equivalent), conductivity of salts and heavy metal contents of the materials to be land-farmed.
The MB sites were drilled and the cuttings processed from October 2005 to May 2006, and BY wells from November 2007 through May 2007.
Land-farming (or any type of remedial cuttings treatment) had not been used before in Bangladesh. Local TPH regulations stipulated a maximum 10% by weight for application to land. However, given Chevron’s commitment to environmental stewardship, a 1% TPH value was placed as the primary objective and key performance indicator (KPI). Use of local personnel as part of the rig site workforce was also identified as a key component of the project. As the drilling campaign was to be an evolving process, with lessons learned leading to improved practices, various KPIs were set up to show the improvement as each remediation site was designed and closed. Chevron and the local Department of Environment (DOE) then accepted the newly introduced technology
For each bioremediation location, additional goals and KPIs were set to control and achieve the primary project goals. These included: spread, treat and process the cuttings and sludge from each independent well; biodegrade the TPH component of the cuttings via stimulating growth of the natural microbial community through nutrient additions and controlled conditions; and to return the location to an environmentally clean and unpolluted state.
The following outlines the methodology and control processes that contributed to the success of this cuttings utilization land-farming project.
Design and Procedure Approach
Drilling fluid selection
• Base fluid. The biodegradability of numerous types of oil-based fluids has been extensively tested and researched, more so over the past decade as the environmental sensitivity of drilling fluid design has fallen under a more intensive spotlight. Data have shown that branching and increasing aromatic content can reduce the potential biodegradability of a base fluid, while isomerised olefins (IO) and linear paraffin (LP) base fluids exhibit increasing biodegradability.
Previous toxicity test protocols have showed a similar pattern of results, with a highly branched diesel and ester performing poorly compared with the IO, which in turn was outperformed by the LP.
From a drilling performance standpoint, the selected base fluid had to exhibit a resistance to elevated temperature and have a rheological profile that would enable maximum hydraulic performance. Such criteria can be defined by measurements like low shear rate viscosity (LSRV), hydraulic horsepower (HHP), equivalent circulating density (ECD) and annular velocity (AV). Each is linked to the rheology of the drilling fluid and will affect the drilling rate and the fluid’s hole-cleaning capacity or solids-removal capacity. The maximisation of the latter parameter is vital when drilling directional or ERD wells in order to avoid drilling problems.
Ester-based drilling fluids are well known for their high biodegradation potential but are susceptible to calcium and acidic gas contamination, as well as thermal limitations. Diesel-based fluids have a naturally high aromatic content and the lowest biodegradation potential. They also can be rheologically limited with potentially high ECDs and limited HHP at the drilling bit.
Based on the higher drilling performance criteria, better biodegradability, lower relative toxicity and the fastest degradation potential over time, a predominantly linear and moderately branched aliphatic hydrocarbon was selected as the base fluid.
• Weighting material. Barite was chosen over calcium carbonate and hematite as the weighting agent. Hematite has the advantage of being able to enrich iron-deficient soils, but it’s also abrasive and can cause significant wear on surface rig equipment and sensitive downhole instrumentation. Logistical re-supply problems also were possible over an extended drilling campaign in remote locations.
Calcium carbonate is a less damaging additive and can contribute to drilling fluid performance, especially in the reservoir sections, and would be a sound, if unspectacular, contributor to soil enrichment. However, as calcium carbonate has a lower specific gravity than either barite or hematite, the amount of solid material required to reach an elevated system mud weight (i.e., >12.5 ppg) can lead to a much higher rheological profile, again adversely affecting ECD and HHP.
Barite contributes little to soil enhancement but has good supply logistics and weighting agent properties; this allows good control of solids content/flow properties and therefore maximises drilling performance.
• Emulsification package and further additives. A sizable range of biodegradable emulsifiers exists, most of which can contribute to good overall standard mud properties. Usually, prior laboratory testing is needed to customise the best combination of candidate salts, alcohols, glycols and glycerine-based additives. For this fluid formulation, a biodegradable emulsifier package was selected that exhibited temperature stability up to 300°F.
An organophyllic clay was used to tailor the drilling fluid system rheology. As a natural montorillonite-based additive, it had potential for additional soil enhancement in sand soils but is only present in small amounts. The internal phase selection of the drilling fluid was based on a calcium chloride brine, thus the total amount of chlorides had to be kept to an absolute minimum that would limit negative impact on soil enrichment but still be sufficient to ensure the internal phase water activity (Aw) was at balance and inhibit any drilled shale reactivity.
Cuttings, land-farming management
Land-farm design and procedures
Prior to initiation of each biodegradation land-farming process, the physical, chemical and biological factors of the environment and of the material to be degraded were evaluated.
Obtaining the most appropriate location for a land-farming location (cell, zone or pan) involved evaluating parameters such as topography, hydrology and rainfall, geology and soil, microbiologic characteristics and waste characterization.
For the MB and BY projects, four sites were identified as the receiving remediation zones. To enable analysis comparisons and validate a non-impact process, baseline sampling was performed prior to any activity, followed by intermediate and closure analysis.
1. The mud components used were identified in the quality assurance and quality control (QA-QC) plan, with SBM being the mud system used for the lower drilling sections. Chlorides (CaCl2) were used in the internal phase of the drilling fluid and did not impact the remediation process or the surrounding environment.
2. Topographical conditions were observed and soil samples were taken at the predetermined location for land-farming. Original soils samples were documented.
3. All engineering design recommendations were for full-scale treatment that reflected specific areas to be addressed on the new location (e.g., area requirement limitations, nutrients needs and expected duration of treatment).
On the rig-site location, the solids control and waste management equipment and processes were designed to allow for cuttings to be collected and segregated into separate pits of WBM cuttings and SBM cuttings. This was important to allow traceability of the products during the degradation program.
The land-farming zones or cells were designed for the total containment of the cuttings and exterior rainfall exclusion. This would maintain control of the remedial process and mitigate potential leaching and surface flows of SBM runoff.
One 2-4 cu m pit was constructed at the bottom end of the remediation SBM zone to allow the internal surface water to drain into this pit. Given that the nature of the hydrocarbon component (linear paraffin) is readily degradable, the pit acts not as total containment but a visual guide to view any surface SBM that may be included with surface water flows. If a TPH surface “film” is observed, the containment pit would be emptied, with the contents being returned to the top end of the SBM zone and used as irrigation supply.
The process required continuous aeration of the cuttings material by mechanical means. Utilization of agriculture tractor units, mechanical rotary hoes, local labour (or combinations) tilled the material into the soil surroundings. Nutrients (primarily nitrogen and carbon components) where combined to ensure the rapid growth of bacterial flora. The existing top soil was removed from each area and used for construction of the surrounding inclusion and exclusion berms. A specified volume of top soil was held back for a combination of amendments and provided the base microbial culture for the remediation process.
The land-farming operation was managed and controlled under a five-phase program:
Phase 1: preparation of the pan
Demarcation and fencing of the remedial zones occurred in conjunction with the operator. All equipment and additives were sourced and commissioned prior to completion of drilling activities. An embankment was built around the pan. About 24 in. of the top soil was removed for the MB5 location to allow for equal volumes of cuttings/amendments/soil to be returned to the location. A 4-in. depth of top soil was mixed with the cuttings and additives for the BY program to enhance the tilling aeration process and reduce the TPH degradation time.
Phase 2: transfer and spread cuttings on the pan
Preparing the pan and transferring the SBM cuttings were done simultaneously in order to efficiently utilize laborers and equipment. While the cuttings were spread on the prepared pan, the remaining part of the pan was prepared for the next cuttings.
Phase 3: combine top soil, rice hulls and NPK fertilizer
The biological treatment of drill cuttings was based on preliminary results from the California Polytechnic State University, San Luis Obispo. This research determined the optimum conditions for bio-treatment.
Based on the initial soil and waste characterization, it was necessary to adjust the proportion of nitrogen, phosphorus and potassium in the remediation cells to maintain the degradation process at optimum levels and to keep the integrity of the micro-organisms species during the active consumption of hydrocarbons. This can be accomplished through correct C:N:P:K ratios.
The efficiency of the bioremediation process is also dependent on conditions such as available oxygen, moisture and nutrients. Conditions must exist where there is greater than 2 mg/l dissolved oxygen or 10% air-filled pore spaces. This was provided by the sawdust/rice hull addition. Targeted parameters were:
Rice hulls and NPK were spread simultaneously during the preparation of the pan and spreading of the cuttings. The addition of fertiliser volumes was pre-determined after establishing the nitrogen percentage of the fertilizer. Fertiliser additions were based on a nitrogen percentage composition of 15%. All fertilizer estimations were based on the nitrogen requirement only. Ratio of cuttings : rice hulls : top soil was 1:1:1. Standard NPK at 4 kg/cu m of cuttings was spread on the pan. This contributed energy sources and nutrients to enhance microbiological activity. Nitrogen values were dependant on the carbon content for correct ratios. Tables 1 and 2 overview the amendment volumetric calculations.
Phase 4: aeration tilling and watering
Dedicated reservoirs for water were prepared at each location. Irrigation of the mixture during the dry season was necessary to maintain sufficient percentage of moisture to maximise the microbial growth, while increasing the biodegradation of the SBM residue. Tillage creates air pockets and voids that will allow a continual aerobic process and promotion of heterotrophic bacteria growth for the hydrocarbon reduction.
Phase 5: decommissioning SBM pits and zone closure
Samples were taken from predetermined areas of each remedial pan and sent for lab analyses. The TPH content was the main parameter to be monitored in the samples. The TPH showed that the process of biodegradation of the hydrocarbons was effective, and the percentage of TPH was gradually decreased to 1%.
The remediated materials were then compacted by backhoe, followed by plate compactor, planted with grass seed, and the site was maintained until grass grew.
Project, process management
The bioremediation project was driven by a quality assurance plan that incorporated a land-farming management plan, compiled by Chevron Bangladesh and M-I SWACO.
Monitoring, analytical procedures
Base soil samples were taken before the bioremediation process to compare results with samples taken during and after the tilling process. Samples were sent for laboratory analyses on TPH, general organic breakdown and primary heavy metals.
Procedures also were followed to ensure there was no potential for environmental impact via surface or ground water. Monitoring holes (piezometers) is the common method to analyse ground water effects. On each remedial location, bore tube wells were drilled and used as indicators of ground water effects on each location.
Daily events were recorded and data collected so the remediation process could be evaluated correctly. All volumes entering the remediation zones and all conditions relating to this and other specific conditions outlined in the quality assurance plan were recorded. A summary document was produced for each location to provide data to satisfy the QA plan and to enable proposals for improvement.
Processed cuttings volumes
MB 4, 5 and 6: A total of 827 cu m of synthetic-based cuttings were processed from 26 October 2005 through 15 May 2006.
BY 1-7: The treated volume of synthetic-based cuttings was 2,379 cu m, from 30 November 2006 to 7 May 2007.
Hydrocarbon, chemical analysis
MB 4, 5 and 6: The TPH declined from the raw material value of +14% to below the targeted 1% and showed that this drilling application and the process of land-farming biodegradation of hydrocarbons was effective. The MB locations were considered closed given the documented successful planning, applied implementation, and monitoring analysis data, which reached the target goals with no outstanding environmental impact.
BY 1-7: Due to early rains in April 2007, the BY bioremediation process was halted at an average TPH of 2.93% for Batch 3 and an average of 2.33% for Batch 4 (Figures 2 and 3). These results show that more time was needed for tilling and aerating this volume of cuttings to lower the TPH content below 1%.
Ground and surface water analysis
Post-activity water samples were taken from the surrounding monitoring wells on all locations. The results showed a low chloride concentration, indicative of the baseline sampling and showing no leachable components had shifted from the remediation pans on any of the MB or the BY locations.
MB5 land-farming pan: The tube well that was used to supply water for watering the pan was also designated as piezometer for this location. Sample results tested at the BY rig indicated that there was no percolation of chlorides on the ground water. A sample taken in February 2006 showed the following results:
MB4 and MB6 land-farming pans: There are two piezometer wells. Piezometer #1 is a bore deep well drilled to supply water during the drilling period (pan 1 for BY4 cuttings). Piezometer #2 is a tube well located near the main entrance security post (Pan 2 for BY6 cuttings). A water pump was used for watering both pans with suction from Piezometer #1. The suction depth for both piezometers is 60 ft.
Parameters (1 Feb, 18 Feb, 1 March and 15 March sample dates)
BY land-farming pans: Similar bore wells were used for all ground water sampling.
The MB 5 land-farming operation was the first to be structured from planning to implementation, with MB4 and 6 following shortly after. The BY project showed the benefits of lessons learned and streamlining processes to bring commercial improvements.
The cost of degradation per cu m (to acceptable TPH levels) increased from MB5 to MB4-6. This was seen as a direct response to having two remediation zones to manage, namely the MB4 and the MB6 remediation pans. There was a dramatic reduction in cost when implementing the BY land-farming project. This is directly attributable to the improvement in processes design, especially the controlled utilization of machinery and subcontractors to reduce daily costs and significantly reduce the number of processing days. The reduction in processing cost from the first MB5 location to the BY project was 57% per cu m.
The increase in cost/day is the result of the larger BY remedial site requiring a higher volume of tillage equipment to process the area. It is expected the cost per cu m required to reduce from the 3% average to the target <1% will be consistent with historical activities and will show a large reduction in the processing costs compared with previous land-farming locations.
A linear paraffin drilling fluid design used in conjunction with the controlled and managed land-farm remediation program can provide an option for a successful drilling and waste management campaign within an environmentally sensitive area.
The overall drilling fluid performance met all reasonable expectations, allowing the successful completion of the Moulavi Bazaar and Bibiyana development wells. General mud properties remained stable, and only moderate amounts of engineering adjustments were needed.
Maximum system mud weights did not exceed 11.1 ppg, and bottomhole temperatures were generally between 190-200°F, with no indication of emulsion instability or water breakout. Some issues were experienced during such an extensive and varied drilling operation, but they were largely attributed to the orientation of certain well path azimuths towards a distinctive field direction. This appeared to result in the wellbore being aligned with either a localised stress fracture pattern, or a certain formation dip, or a combination thereof. In addition, during extended coring operations, fluid loss control was supplemented by additional polymeric materials to ensure minimal initial spurt loss or fluid invasion.
New benchmarks for land well directional drilling also were established in Bangladesh. Bibiyana well BY-10 was drilled to a total depth of 5,053 m (3,356 m TVD) at a maximum angle of 54.2°. The tangent section for this well exceeded 2,800 m.
Within the guidelines of this remedial land-farming project, results indicate that given well managed environmental conditions, local bacteria will reduce the TPH component within a linear paraffin-based synthetic fluid system to below 1%. Factors that contribute to the time of degradation are availability of oxygen, nutrients and moisture, and the frequency of tillage must be considered when establishing and promoting local biota growth.
Local soil quality appears enhanced and promotes growth for reinstatement with the combination of SBM drilled cuttings, carbon sources (sawdust and hulls), fertilizers and notable increased porosity. Ground and surface water quality will not be negatively impacted if mitigation measures are in place.
Processing of SBM cuttings via a remedial land-farm degradation process is not only environmentally agreeable and supports the local community economy through employment, but it also offers a financially viable means to reduce the cuttings to a benign state, reducing liability risks. The processes and monitoring must be measured through key performance indicators and show value from one project to the next.
Acknowledgements: The authors thank the management of PetroBangla, Chevron Bangladesh and M-I LLC for permission to publish this article. Additional thanks to the dedicated operation staff from Chevron and M-I LLC within Bangladesh.
This article is based on a presentation at the IADC Drilling HSE Asia Pacific Conference & Exhibition, 26-27 February 2008, Kuala Lumpur.
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