Resultados del análisis de percepciones Para analizar las percepciones sobre el tema se aplicó una

A prime objective in all drilling operations is to minimize safety and environmental risks while maintaining drilling performance. Operators and service companies alike take a proactive stance to reduce the potential for

Fig. 3.28—Decanting-centrifuge schematic (Annis 1974). Reprinted courtesy of ExxonMobil.

Bowl rotation Feed Overflow liquid discharge Underflow coarse solids discharge

hazardous incidents and to minimize the impact of any single incident. The HSE policies of many companies are more stringent than those required by national governments and the various agencies charged with overseeing drilling operations. All personnel who take part in the well-construction process must comply with these standards to ensure their own safety and the safety of others. On most locations, a zero-tolerance policy is in effect concern- ing behaviors that may endanger workers, the environment, or the safe progress of the operation. Additionally, all personnel are encouraged to report any potentially hazardous activities or circumstances through a variety of observational safety programs.

The packaging, transport, and storage of drilling-fl uid additives and/or premixed-fl uid systems are subject to close scrutiny regarding HSE issues. Personnel who handle drilling fl uid and its components are required to wear personal protective equipment (PPE) to prevent inhalation and/or direct contact with potentially haz- ardous materials. Risk-assessed ergonomic programs have been established to reduce the potential for inju- ries related to lifting and operating mud-mixing equipment.

3.9.1 Protecting the Environment. In addition to the obvious benefi ts of minimizing, if not totally eliminating, sources of pollution and related threats to the ecosystem, the oil and gas industry recognizes that governmental permission to acquire and develop commercial reserves worldwide is more easily obtained if drilling-related ac- cidents are few and far between.

Drilling fl uid companies strive to achieve and maintain an “econoecological” balance with each drilling-fl uid system and additive. A “green” drilling-fl uid system that performs poorly will seldom be used; poor performance extends drilling time and increases the likelihood of hole problems as well as the cost of well construction. Con- versely, using a properly managed high-performance SBF can shorten the duration of the drilling operation and/ or help maintain wellbore stability, thereby reducing opportunities for environmental damage. These and other factors must be weighed in the design and selection of any drilling-fl uid system.

3.9.2 Sources of Contamination. Both land- and offshore-drilling locations are subject to regulations addressing the disposal of whole mud, drill cuttings and other solids, and run-off, if any, generated by rainfall, wave action, or water used at the rigsite. Industrywide efforts to eliminate environmental hazards resulting from accidents or the negligent handling of drilling fl uids and/or drill cuttings encompass several contamination issues related to drilling fl uids:

· Formulation: chlorides, base oils, heavy metals, corrosion inhibitors · Natural sources: crude oil, salt water, or salt formation

· Rigsite materials: pipe dope, lubricants, fuel

In some cases, reformulating drilling-fl uid systems makes them more benign to the environment. For example, chrome lignosulfonate water-based fl uid is available in a chrome-free formulation. The development of SBFs re- sulted from the need to replace diesel and mineral OBFs due to environmental restrictions.

The discharge of conventional OBFs and drill cuttings was effectively prohibited in the North Sea in 2000. Cuttings generated by drilling with certain compliant SBFs may be discharged overboard in the western GOM if they comply with the retention on cuttings (ROC) limits introduced in 2002. Neither traditional OBFs nor the drill cuttings produced while using them can be discharged in the GOM; the rare offshore operation using a diesel- or mineral-based fl uid must include a closed-loop process for continuously capturing all drill cuttings and returning them to shore for regulated disposal.

3.9.3 Drilling Fluids and Waste-Stream Reduction. As a result of the increasing emphasis on environmental protection and effi cient use of resources, the concept of Total Fluids Management® _{(TFM) has emerged as a} means of conserving materials, improving operational performance, and generating the appropriate documenta- tion to verify that a given well-construction project was completed in compliance with governmental and corpo- rate regulations.

The key focus of TFM is to cover the interface between all aspects of the drilling operation and to exploit the natural synergies between different service specialties. The mission of the TFM supervisors is to guide and focus all members of the team on their individual targets in order to achieve the collective goal. The mission also ad- dresses the means and resources required to achieve the goals, measure the improvement, and refi ne the expecta- tions for further project stages. The examples of TFM discussed here represent practices in the North Sea and the GOM.

During the late 1990s, a major Norwegian operator analyzed current practices related to the control and management of drilling and well fl uids and the associated waste generation (Paulsen et al. 2001). A TFM model

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was developed to improve environmental performance by reducing waste, maintaining or improving drilling performance, and preserving the profi tability of contractor and service-company participants. The model was designed to reward performance according to Key Environmental Performance Indicators (KEPIs), so that any reduction in materials usage (i.e., sales) was offset. Once all participants agreed to the criteria, a TFM program was implemented to address the following areas:

· Measurement and management of drilling fl uids, completion fl uids, and drill cuttings · Waste minimization

· Performance assessment · Report formats and protocols

· Scheduled performance and target reviews · Continuous improvement

In the GOM, regulations covering the discharge of synthetic-based drilling fl uids and drill cuttings have increased the complexity, number, and types of measurements along with the potential for noncompliance. This has created the need for checks and balances in the daily monitoring and in the end of well report issued by the environmental testing company on location. The resulting environmental-compliance document must be provided to the operator and must be verifi able and auditable by governmental and environmental oversight groups.

3.9.4 Waste Management. The use of inhibitive drilling-fl uid systems and good performance from solids-con- trol equipment are key factors in the effort to reduce the volume of solids and liquids generated at the rigsite. Studies indicate that the greatest impact on waste-minimization strategies and capabilities is achieved through close monitoring and control of the drilling-fl uid system, though other fl uid-related operations such as comple- tion, cementing, and rig-wastewater handling should also be part of the TFM program. Implementation of these policies resulted in overall well-construction-cost reductions and increased drilling performance.

Because of the inherent value of synthetic-based drilling fl uids and the restrictions applied to their discharge, the reuse of these fl uids has long been an industry standard. The various TFM programs currently implemented worldwide ensure that the recovery of reusable fl uids is optimized through close attention to

· Fluid quality assurance/quality control (QA/QC) · Fluid transport

· Fluid fl ow at the rigsite

· Solids-control-equipment operation · Cuttings processing

Documentation requirements for these processes provide quantifi able indicators of environmental and economic performance of a specifi c TFM program.

In areas where drill cuttings must be returned to shore, recently improved pneumatic-transfer systems allow the cuttings to be captured below the shale shakers and pumped with compressed air to rigsite-storage units (Martin 2002). Supply boats equipped with the same pneumatic system receive the cuttings from the rig, transport them to shore, and then pump them into large storage tanks for eventual disposal. Auger-type conveyances are also commonly used to move drilled cuttings from the rig to vessels for treatment and/or transport to shore.

3.9.5 Treatment and Disposal of Drilled Cuttings. Where offshore discharge of cuttings is not sanctioned by regulatory and environmental agencies, there are many possible disposal options. Waste segregation at the rigsite is a key factor in the success of zero-discharge programs. Drilling-fl uid waste, drill cuttings, waste generated by cementing or completion operations, and waste water from the rig must be handled and stored separately.

Another option is minimizing the initial volume of cuttings through installation and careful maintenance of high performance solids-control equipment. An effective cuttings-drying process should be established at the rigsite so that any cuttings transported to shore have the lowest possible fl uid retention. At present, thermal de- sorption is considered one of the most effective methods of processing cuttings. Thermal desorption results in a 0.1 to 0.5% ROC rate and recovers the liquid oil for possible reuse. In thermal desorption, cuttings are heated to a very high temperature so that oil and water are boiled into gases. The water is released as steam and the oil is condensed to liquid.

However, the recovered oil may not have the same properties as the original base fl uid because of the possi- bility of high temperatures breaking the heavier hydrocarbon molecules into lighter compounds. This may lower the oil fl ash point and alter its rheological properties. Though these changes are generally slight, they are

suffi cient to prevent most operators from attempting to reuse the oil in a drilling-mud formulation. The oil is typically used as heating oil or to fuel the thermal desorption process. Salts or heavy metals are not removed from the cuttings.

3.9.6 Encapsulation. Recommended primarily for the treatment of oil-wet cuttings (but also effi cient in solidi- fying liquid oil-based mud, water-based-mud cuttings, or solids discharge from fl occulation systems), the typical encapsulation process is capable of treating up to 20 bbl/hr. The cuttings are mixed with calcium or silicate mate- rials to produce a low-permeability cementitious matrix that reduces the mobility of oil. Kiln dust, an inexpensive waste by-product of cement factories, can be used, as can standard cement. The generator-powered system con- sists of one hopper for cuttings/sludge and one hopper for chemical treatment. Both feed to a central mixing area by means of screw conveyors. The speed of each conveyor can be adjusted, allowing the operator to control the ratio of chemical to sludge (generally from 30 to 50 % by weight).

A silica-calcium oxide-encapsulating agent is mixed with the raw cuttings and thorough mixing is ensured. Water is added as demanded by the process. The cuttings are recirculated in the mixing device (auger, auger tank, and ribbon blender) until satisfactory appearance is observed and fi eld scale tests (sheen/can test) are passed. Specialized laboratory tests are required and compliance with the applicable regulations ensured before following a customer’s instructions for disposal.

3.9.7 Bioremediation. Bioremediation is the process of using micro-organisms in a controlled, engineered envi- ronment to reclaim soil, sludge, and water polluted by hazardous and nonhazardous substances that can affect human health and/or the environment. These micro-organisms may be native to the contaminated media, geneti- cally developed/enhanced, or they may be isolated from natural processes, selectively adapted to degrade a spe- cifi c contaminant and brought to the contaminated site.

Land farming is the use of native bacteria, helped with additions of nutrients, water, and aeration, to break down harmful substances into environmentally-safe compounds. The broad metabolic capabilities of the micro-organ- isms enable them to remove or to reduce pollutant concentrations to levels that no longer present a risk to human health or the environment. In addition, the use of micro-organisms is not capital intensive, making bioremediation a cost-effective and feasible solution for small- and large-scale applications.

For bioremediation to successfully occur, the contaminants in question must be degradable by the involved micro-organisms. The breakdown of these contaminant molecules is accomplished by enzymes produced by the microbes. Enzymes can be added to the solids to increase the rate of breakdown.

Hydrocarbons are biodegradable. N-alcanes and n-alquilaromatics between C10 and C22 are generally con- sidered low toxic. Compounds between C5 and C9 are biodegradable at low concentrations. Gaseous alcanes from C1 to C4 are biodegradable, but this is not the usual removal mechanism. Alcanes, n-alquilaromatics, and aromatics over C22 show low toxicity, but their physical characteristics, including their low solubility in water and their solid state at 35°C (optimum bioremediation-process temperature), are a problem, affecting their biodegradability.

Biodegradation is not always a successful process; sometimes the substance remains unaltered for one or more of the following reasons:

· The chemical concentration is high enough to kill micro-organisms.

· The number and type of micro-organisms is inadequate to carry out the process. · The soil acidity or alkalinity may be inappropriate.

· The micro-organisms may suffer from a lack of nutrients such as nitrogen, phosphorous, potassium, sul- phur, or other micronutrients necessary for their normal metabolism.

· The moisture conditions may be inadequate.

· The micro-organisms may suffer from a lack of oxygen, nitrate, or sulphate, which are their main energy source.

Bioremediation is one of the most cost-effective treatment methods available for destroying certain categories of hazardous and nonhazardous waste, offering the following advantages:

· Proven technology for on-site destruction of many organic contaminants to concentrations below the cleanup standards

· Cost savings associated with the on-site treatment and low-capital cost (typically requires simple and read- ily available equipment)

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· Flexibility to perform treatment on-site rather than transporting cuttings to a central facility

· The ability to use treated material as fi lling material or base material to be mixed with untreated cuttings

Certain considerations apply to the use of bioremediation. These include high levels of oxygen, which facilitate bacteria growth. In the case of land farming, the soil is aerated by mechanical tilling of the soil. The biodegrada- tion rate is directly proportional to the oxygen level in the soil.

The bioremediation process can be adversely affected by high rain conditions; therefore it is necessary to install appropriate drainage systems to direct, collect, use, and/or dispose of rain water. Proper roofi ng should be pro- vided for bioremediation facilities. The fi eld capacity is the amount of water held in the soil after the excess gravitational water has drained away and after the rate of downward movement of water has materially decreased. A moisture level between 50 and 70% is best for bioremediation.

The optimum temperature for bioremediation ranges from 75°F to 98°F. Lower temperatures slow or stop bac- teria activity, while temperatures above 135°F can destroy most bacteria. The optimal pH range for hydrocarbon- eating bacteria is from 6 to 8. Below 5, fungi become the predominant soil microbe and the bacteria population is reduced. A pH greater than 6 is recommended to minimize hazardous metal migration.

Bioreactors are designed to reduce the cuttings to a benign soil-like material that can be disposed of without risk at any onshore location. Small bioreactors have been developed for use in laboratories, but at present there are no units with suffi cient capacity to handle the volumes of cuttings typical of offshore drilling operations.

Problems

3.1 Discuss the functions of a drilling fl uid.

3.2 An 11.4-lbm/gal freshwater mud is found to have a solids content of 16.2 vol%. (a) Compute the volume fraction of API barite and low-specifi c-gravity solids.

Answer: 0.068 and 0.094.

(b) Compute the weight fraction of API barite and low-specifi c-gravity solids in the mud.

Answer: 0.209 and 0.179.

(c) Compute the API barite and low-specifi c-gravity solids content in pounds per barrels of mud.

Answer: 100 and 85.5 lbm/bbl.

3.3 Compute the density of a mud mixed by adding 30 lbm/bbl of clay and 200 lbm of API barite to 1 bbl of water.

Answer: 11.8 lbm/gal.

3.4 Determine the density of a brine mixed by adding 150 lbm of CaCl₂ to 1 bbl of water.

Answer: 10.7 lbm/gal.

3.5 Discuss the desirable and undesirable aspects of a high mud viscosity.

3.6 Compute the yield of a clay that requires ad dition of 35 lbm/bbl of clay to 1 bbl of water to raise the apparent viscosity of water to 15 cp (measured in a Fann viscometer at 600 rev/min).

Answer: 59.3 bbl/ton.

3.7 A mud cup is placed under one cone of a hydrocyclone unit being used to process an unweighted mud. Twenty seconds were required to collect 1 qt of ejected slurry having a density of 20 lbm/gal. Compute the mass of solids and water being ejected by the cone per hour.

Answer: Solids: 852 lbm/hr and water:47.6 lbm/hr.

3.8 A 1,000-bbl unweighted freshwater-mud system has a density of 9.5 lbm/gal. What mud treatment would be required to reduce the solids content to 4% by volume? The total mud volume must be main- tained at 1,000 bbl and the minimum allowable mud density is 8.8 lbm/gal.

Answer: Discard 544 bbl of mud, add 544 bbl of water.

3.9 The density of 600 bbl of 12-lbm/gal mud must be increased to 14 lbm/gal using API barite. One gal- lon of water per sack of barite will be added to maintain an acceptable mud con sistency. The fi nal volume is not limited. How much barite is required? Answer: 92,800 lbm.

3.10 The density of 800 bbl of 14-lbm/gal mud must be increased to 14.5 lbm/gal using API barite. The total mud volume is limited to 800 bbl. Compute the volume of old mud that should be discarded and the weight of API barite required.

Answer: Discard 19.05 bbl of mud, add 28,000 lbm of barite.

3.11 The density of 900 bbl of a 16-lbm/gal mud must be increased to 17 lbm/gal. The volume fraction of low-specifi c-gravity solids also must be reduced from 0.055 to 0.030 by dilution with water. A fi nal mud volume of 900 bbl is desired. Compute the volume of original mud that must be discarded and the amount of water and API barite that should be added.

3.12 Assuming a clay and chemical cost of USD 10/bbl of mud discarded and a barium sulfate cost of USD 0.10/lbm, compute the value of the mud discarded in Problem 3.11. If an error of + 0.01% is made in determining the original volume fraction of low-specifi c-gravity solids in the mud, how much mud was unnecessarily discarded?

Answer: USD 16,697; 191 bbl.

3.13 Derive expressions for determining the amounts of barite and water that should be added to increase the density of 100 bbl of mud from ρ₁ to

ρ2. Also derive an expression for the increase in mud volume expected upon adding the barite and the water. Assume a water requirement of 1 gal per sack of barite.

Answer: M_B = 109,000 (ρ₂ − ρ₁)/(28.08 − ρ₂); V_w= M_B / 4,200; V = 0.0091 M_B.

3.14 A 16.5-lbm/gal mud is entering a centrifuge at a rate of 20 gal/min along with 8.34 lbm/gal of dilution water, which enters the centrifuge at a rate of 10 gal/min. The density of the centrifuge underflow is 23.8 lbm/gal while the density of the overfl ow is 9.5 lbm/gal. The mud contains 25 lbm/bbl ben-