Dewatering

Dewatering is the process of removing excess water from spent drilling fluid or drilling waste to reduce the volume that must be disposed of and to separate recyclable solids from the liquid. In drilling operations, dewatering refers specifically to the treatment of waste water-base mud (WBM) at the end of a well or during operations when the mud system accumulates too much water. The process typically involves a chemical treatment step (adding polymers to cause fine solids to flocculate into larger aggregates) followed by mechanical separation (a filter press, centrifuge, or vacuum belt filter) that squeezes most of the water out of the flocculated solids. The recovered solids go to a landfarming or landfill site; the recovered water may be recycled, discharged to surface water under a permit, or reinjected into a disposal well.

Key Takeaways

Dewatering reduces waste volume dramatically. A typical spent WBM with 70 to 80 percent water content can be reduced to 30 to 40 percent water in the filter cake, cutting the volume of material to be transported and disposed of by half or more. This directly reduces disposal trucking costs and disposal facility fees.
Chemical flocculation is the first step. A high-molecular-weight polymer (polyacrylamide or polyelectrolyte) is added to the mud while mixing. The polymer chains bridge across fine clay and barite particles, causing them to aggregate into large, rapidly settling flocs. Without this step, the fine colloidal particles in drilling mud pass through mechanical filters.
Mechanical dewatering equipment includes filter presses (a series of filter plates that squeeze the flocculated mud under hydraulic pressure), centrifuges (which spin the mixture at high RPM to separate solids from liquids by density difference), and belt filter presses (where the flocculated mud is continuously fed between two porous belts that squeeze it).
Dewatering of barite-weighted mud is complicated because barite (barium sulfate, a dense weighting agent) must be recovered and recycled if possible, as barite is expensive and its disposal requires proper characterization. Some dewatering systems include a barite recovery step using centrifuges to separate the high-density barite from the lighter clay fraction.
The Alberta Energy Regulator (AER) Directive 050 governs drilling waste management in Alberta, including dewatering operations. Operators must characterize the dewatered solids and liquids by testing (for sodium, boron, chlorides, hydrocarbons, and other parameters) to determine the appropriate disposal method.

What Is Dewatering and Why Is It Done?

After a well is drilled, the mud system that was used to drill it is no longer needed. If the mud is a water-base system (which is most common for non-reactive shale sections), it contains enormous volumes of water mixed with fine clay, barite, and chemical additives. A typical 3,000-metre well drilled with WBM might end with 500 to 1,000 cubic metres of waste mud in the pits. That is the equivalent of 500 to 1,000 cubic metres of liquid waste, most of it water.

Disposing of 500 to 1,000 m³ of liquid waste is expensive. Trucking it to a licensed disposal facility costs roughly CAD 25 to 50 per cubic metre including transport. Dewatering first, then disposing of only the solid portion, reduces the volume by 40 to 60 percent. The economics are straightforward: spend a smaller amount on dewatering to save a larger amount on disposal fees and trucking.

The recovered water can often be reused in the next drilling program (reducing fresh water consumption), discharged to surface water under an environmental permit if it meets quality standards, or disposed of by land application if the chemical content is within agricultural tolerances. The solid filter cake, which is drier and easier to transport than the original liquid mud, goes to an engineered waste disposal site or a land treatment facility where the organic content can be biodegraded.

Fast Facts

Mobile dewatering units are available as trailer-mounted equipment that arrives at the rig site when a well is completing, processes the entire mud system over several days, and then moves to the next location. Service companies including Newpark Drilling Fluids, Halliburton Baroid, and Baker Hughes offer dewatering services as part of their drilling waste management business. In Canada, Environmental 360 Solutions and Clean Harbors are major waste management contractors that operate mobile dewatering equipment for the oil and gas sector in Alberta and Saskatchewan.

The Dewatering Process Step by Step

Dewatering begins with an assessment of the mud system's properties. The mud engineer measures the volume, density, pH, chloride content, and solids content of the waste mud. This information determines how much polymer is needed for flocculation and whether any pre-treatment is required (for example, diluting a high-chloride mud that might otherwise interfere with the polymer).

The polymer (typically a high-molecular-weight anionic polyacrylamide) is prepared as a dilute solution and injected into the mud flow at a controlled rate using a mixing tee or static mixer. The polymer concentration needed depends on the clay content of the mud: higher clay content (more colloidal particles to aggregate) requires more polymer. A typical dose is 0.5 to 5 grams of polymer per cubic metre of mud.

The flocculated mixture flows to the dewatering equipment. In a filter press, the flocculated mud enters the chambers between filter plates and the hydraulic press squeezes the plates together. Water passes through the filter medium (cloth or membrane) and drains away; the solids are retained as a filter cake. After pressing, the plates are opened and the cake is discharged. A filter press can process 5 to 30 cubic metres of mud per hour depending on size and filtration characteristics.

The filter cake is tested for chloride, hydrocarbons, and heavy metals before disposal. In Alberta, this testing follows AER Directive 050 guidelines, which specify the tests required and the thresholds that determine whether the solid waste can go to a Class I landfill, a Class II landfill, or requires treatment before disposal.

Dewatering in Coalbed Methane and Water-Intensive Operations

Coalbed methane (CBM) wells in the Horseshoe Canyon and Mannville formations of Alberta produce very large volumes of formation water along with gas. Unlike oil wells where produced water is often reinjected, CBM-produced water at some locations has been treated and used for agricultural irrigation under AER approval. The water treatment process for CBM-produced water is analogous to drilling mud dewatering: remove fine solids, reduce total dissolved solids, and test before any disposal or beneficial use.

Hydraulic fracturing operations also generate flowback water that contains sand, formation fines, and fracturing chemicals. Some operators dewater the flowback sand to separate the sand from the water before disposing of each fraction through the appropriate pathway. Dewatering of fracturing flowback is a growing segment of oilfield water management in the Montney, Duvernay, and Cardium plays of Alberta.

Dewatering is also called liquid-solid separation in drilling waste management contexts. Related terms include filter press (a mechanical dewatering device consisting of a series of filter plates that squeeze flocculated drilling waste under hydraulic pressure to remove water; produces a solid filter cake suitable for solid-waste disposal), flocculation (the aggregation of fine colloidal particles into larger clusters (flocs) by the addition of a polymer or coagulant; the chemical treatment step that makes fine drilling mud particles large enough to be captured by mechanical dewatering equipment), centrifuge (a rotating drum that separates solids from liquids by centrifugal force; used in drilling for barite recovery and dewatering, and in produced water treatment for oil-solids separation), drilling waste (the spent mud, cuttings, and contaminated fluids generated during the drilling of a well; subject to characterization and disposal requirements under AER Directive 050 in Alberta and equivalent regulations in other jurisdictions), and water-base mud (a drilling fluid in which water is the continuous phase; the dominant mud type for non-reactive formations; generates large volumes of aqueous waste that require dewatering before disposal).

How Dewatering Saved a Saskatchewan Operator CAD 420,000 on a Single Well Program

A mid-size Saskatchewan operator was drilling a four-well Bakken program in southeast Saskatchewan using potassium chloride (KCl) water-base mud. By the end of the fourth well, the mud system contained 780 cubic metres of waste mud at approximately 75 percent water content. The mud was contaminated with formation clay and drill cuttings from the Bakken and underlying formations.

The original disposal plan was to truck the liquid waste to a licensed disposal facility 95 kilometres from the drilling location, at a cost of CAD 42 per cubic metre for trucking and facility fees. Total disposal cost for 780 cubic metres: CAD 32,760.

The drilling supervisor reviewed the dewatering option. A mobile filter press unit arrived at the location for CAD 28,000 in mobilization and three days of operation. The dewatering reduced 780 cubic metres of liquid waste to 310 cubic metres of filter cake (drier solid) and 470 cubic metres of recovered water. The recovered water, testing within AER Directive 050 parameters for KCl-treated water, was retained in a lined pit for use in the next program. The filter cake was trucked to the disposal facility: CAD 13,020 in trucking and fees for the 310 cubic metres. Total dewatering + disposal cost: CAD 41,020 versus CAD 32,760 without dewatering. But the 470 cubic metres of recovered water saved CAD 11,750 in fresh water procurement for the next program. Net saving versus full liquid disposal: approximately CAD 3,490 on this well program alone. For operators running 40 to 60 wells per year, the savings scale significantly.