Dewatering and Recycling Sump Drilling Mud

INTRODUCTION

The dewatering unit (mud recycling unit) was introduced to reduce the high disposal/transportation cost by processing and recycling the sump drilling fluid.

A dewatering unit (mud recycling unit) was introduced at MWSS, in an effort to reduce the high disposal cost of sump fluid. Disposal cost for non-hazardous liquid had risen from $10.10/bbl to $16.40/bbl. Mud disposal cost represented 15% to 20% of the cost to drill and complete a MWSS well. Dewatering and recycling sump drilling mud reduced this disposal cost to $3.49/bbl and, thereby, lowered drilling costs by $11,621/well (average). Continued utilization of this process on 141 wells generated a savings of $1,638,561.The development drilling program at MWSS Field consisted of two drilling rigs generating approximately 17,000bbls of sump fluid per month. Environmental regulations required trucking the sump fluid to a state-approved disposal site approximately 25 miles away. Disposal costs were averaging approximately $10.10/bbl, or $170,000/month, initially and represented 15% of the cost to drill and complete a MWSS well.

BACKGROUND

Certain limitations are inherent in the design of mechanical solids separation equipment. The size of the particles removed will be contingent upon specific gravity of the solids, viscosity of the fluid, G-force developed, and a host of other variables. Without chemical enhancement, high speed centrifuges are limited to a 2-3 micron cut.

However, the objective at Sun Exploration and Production Company’s MWSS lease is to provide a “solids-free” effluent for recycling after processing the waste drilling fluid. Clearly, steps to enhance the performance of the centrifuge were required.

To overcome the limitations of the machine, the feed mud is pretreated chemically to increase the “effective particle size” of the suspended solids. This flocculation process enables the centrifuge to remove essentially all of the suspended solids from the feed slurry.

DESIGN AND INSTALLATION

The drilling mud processing unit is a stationary installation, centrally located in the MWSS Field that removes suspended solids from sump drilling mud through chemical treatment and mechanical separation. This process yields a liquid effluent which is recycled back into the drilling fluid system on new wells. The separated moist solids are dried and mixed into the surface soil.

The location for the processing unit was selected to minimize trucking distance from the reserve pits and to take advantage of the 460V electrical supply on the lease. Figure 1 describes the site layout and outlines the operation of the unit. Drilling fluid (fresh water/polymer mud) is hauled by vacuum trucks from the reserve pits to the processing unit and unloaded into an earthen sump. The mud is agitated with mixing guns to maintain the solids in suspension. Larger solids are mechanically removed by a fine screen shaker as the waste mud is pumped into a 500-bbl storage tank (Figure 2). Further agitation is required to prevent the suspended and colloidal solids from settling in the waste mud tank. Another 500-bbl tank contains fresh water and is used for chemical hydration (Figure 3). The waste mud is then pumped into the dewatering unit (mud recycling unit) (Figure 4). The pH of the waste mud is first measured and reduced to a range of 6-9, if necessary, by additions of HC1. At this point, the actual chemical/mechanical process begins.

The dewatering process can be divided into three stages: delivery, blending, and separation (Figure 5). Drilling fluid from the agitated storage tank and “activated” polymer are pumped separately to the blending manifold. Flow rates are monitored with turbine-type flow meters, and cumulative volume processed is recorded daily. The blending manifold is designed to systematically inject and blend the drilling fluid, recycled effluent (if necessary), and the flocculants (polymers) to produce a flocced slurry that flows directly to the centrifuge. Separation of solids from the feed slurry occurs in the decanting centrifuge. This final mechanical separation, assisted by chemical flocculation, creates a clarified liquid virtually free of suspended solids (200-1200 ppm) and moist solids discard (35-45% water by weight).

Figures 6 and 7 show the dewatering unit (mud recycling unit) in operation. As seen in the photographs, the moist solids are transported away from the unit by a conveyor belt. The solids are then dried, spread, and mixed into the surface soil. Laboratory results have shown the processed solids to be nonhazardous material (relative to toxicity, ignitability, reactivity, and corrosives) and comply with current California environmental regulations for burial.

The recycled water is stored in three 500-bbl tanks and is reused as makeup water in the continuous development drilling program. Processed water is not used for rig water (radiator coolant) or makeup water for cementing to prevent potential compatibility problems.

FIELD PERFORMANCE

Table 1 identifies the mud processing activity and volume schedule from October, 1986, through September, 1987. As shown, a total of 170,412bbls of sump fluid were processed from 141 MWSS wells during this twelve-month period of time. This represents an average mud volume of 1,209bbls/ well. The processed volume ranged from approximately 9,000-20,000bbls/month.

Several field practices adopted at MWSS during this processing period resulted in improved system efficiency.

• Pilot tests on actual representative waste mud samples are necessary to determine the most cost-effective chemical treatment(s). The selection of the “best” flocculant is based on the size, durability, and compressibility of the floes produced and the clarity of the liquid phase.
• Early removal of larger solids fine screen shaker reduced the flocculant required in the process. with the amount of dewatering
• Installation of mixing guns in the earthen pit and waste mud tank is necessary to prevent suspended particle settling and to provide a homogeneous fluid to the dewatering unit (mud recycling unit).
• Cement-contaminated waste mud required pH adjustment prior to polymeric treatment.
• The quality of the dewatering process is easily controlled visually at the centrifuge effluent discharge point, but requires experienced operator attention. An insufficient flocculant feed rate or irregularities in the waste mud composition result in “dirty” effluent and require immediate response. A change in the mud chemistry or solids content of the waste mud can necessitate a change in the polymer selection or combination of different polymers.

COST COMPARISON

The present MWSS disposal cost ($60/ton of nonhazardous liquid) is calculated and contrasted to the processing cost in Table 2. As shown, the present disposal cost, inclusive of trucking and truck washout fees, is estimated at $16.40/bbl or $19,828/well. Current processing cost—including equipment rental, chemical, in-field trucking, and personnel–averages $3.49/bbl, or $4,219/well.

In addition to this substantial cost reduction ($15,609/well currently), another economic benefit was achieved through mud processing at MWSS by eliminating disposal. The soluble polymer from the original low solids/polymer mud system was retained in the processed fluid, which lessened mud costs on subsequent wells. As this processed fluid is recycled as makeup water, the amount of additional polymer required to build the mud system is slightly reduced. Analysis of computerized mud cost summary data for the Anderson-Goodwin lease at MWSS prior to the installation of the dewatering unit (mud recycling unit) established an average mud cost of $1,985/well (normalized to 1987 prices). The average mud cost during the twelve months of dewatering operations was $1,843/well–a savings of $142/well (Table 3).

Makeup water transportation cost at MWSS was essentially the same whether recycled water or fresh water was utilized. The actual water and transportation cost with mud disposal and mud processing averaged $1,938/well and $1,903/well, respectively (Table 3). However, dewatering unit (mud recycling unit) applications in other areas could create significant savings through reduced makeup water transportation cost. The reduction in the combined mud and water transportations costs amounts to 1.12% of the total savings–and, in this case, is relatively insignificant.

The weighted average disposal cost/barrel for the twelve-month period was $13.11 (see Table 4 and Figure 8). Utilization of the dewatering system at MWSS reduced the average cost of each well drilled by $11,621. (Table 5).

CONCLUSION

The dewatering process (recycling processing) is alternative to hauling and hazardous waste drilling fluid.

Table 1-5