How to Feed Gas Plants with Uncontaminated Effluents and How to Avoid Environmental Pollution During Testing/Cleaning Operations of Newly Drilled/Worked-Over Gas Wells
Authors: Ahmed Ali Moin 
Published: May 2016

This reference is for an abstract only. A full paper was not submitted for this conference.

Introduction

Gas plants are normally designed to receive/treat clean natural gas streams free of residuals of high salinity water and solids (such as: black powder, cement particles, drilling cuttings, etc.). Gas wells produce normally condensed water of very low salinity (&lt; 200 mg/l). Surroundings of the pay zone down-hole sections of newly drilled and/or worked-over gas wells are normally contaminated with filtrate residuals of drilling/completion fluids as well as with water-based fluids probably lost during drilling/work-over operations. Therefore, produced gas stream ex newly drilled/worked-over gas wells are normally associated with mixtures of condensed water & some quantities of high salinity water (up to 200000 mg/l) along with some solids. i. e., each of such gas wells needs probably more than one month (or more) of costly cleaning operation, before flowing to the gas plant for further smooth treatment processes. During the cleaning operation, it is recommended to minimize polluting the environment (in as much as possible) by avoiding flaring of produced hydrocarbons into the atmosphere. During cleaning operations of newly drilled/worked-over gas wells, it is proposed to carry out pilot trials on flowing the natural gas/condensate streams through liquid: gas & liquid: liquid coalescence cartridges before flowing to the gas plants for further smooth treatment processes without the need to be flared into the atmosphere. Flowing contaminated gas with residuals of drilling/completion fluids through on-line liquid: gas & liquid: liquid coalescence cartridges after pre separation of bulk liquids & solids could probably help in separating these contaminating waters/solids and allowing the clean natural gas/condensate streams flow to the gas plant.

Possible Sources of Contaminations in Produced Fluids Ex Newly Drilled/Worked-Over Gas Wells

Surroundings of pay zone hole sections in newly drilled/worked-over gas wells are normally contaminated with filtrate residuals of drilling/coring/completion/work-over fluids, acid stimulation mixtures and cement slurries (in case of production from perforated casings/liners) due to following:Sodium chloride (PVD salt) is mainly used to prepare drilling/coring/completion/work-over fluids of density up to 10 ppg. Drilling fluids filtrate will be rich with calcium & sulfate ions when anhydrite streaks are encountered in the pay zone hole section. Therefore, filtrate of non-damaging drilling/coring/ completion/workover fluids (of density up to 10 ppg) could contain chloride & sulfate ions up to 150000 & 10000 mg/l, respectively.Invasion & migration of loose pay zone particles, oil well cement/acid soluble cement pieces, LCM, weighing materials, upper formation loose particles behind the casing, etc.Hydrochloric acid mixtures of HCl concentrations up to 28 % are normally used to stimulate oil & gas wells in carbonate reservoirs. Residual of spent acid mixtures could contain chloride & calcium ions up to 200000 & 50000 mg/l, respectively. Residual of spent acid mixtures could also contain iron hydroxide as well as trace of acid corrosion inhibitors, gelling agents, etc.When pay zone hole sections are covered by perforated casings or liners, surroundings of these hole sections could be contaminated with free solid particles (example: 1), as well as with drilling fluids/cement slurries filtrate residuals those are rich with chloride, sulfate & calcium ions.In case of lost circulation during drilling operations in the pay zone, surroundings of the hole sections would be contaminated with probably more than 10000 barrels of location water or sea water those are rich with chloride & sulfate ions.

Requirements for Cleaning Newly Drilled/Worked-Over Gas Wells

Natural gases are saturated with water vapor at reservoir conditions of temperature & pressure. When these gases are produced, water vapor condenses as temperature decreases gradually from bottom hole static temperature (BHST) down to the well head temperature. Chloride content in condensed water is normally very low (&lt; 100 mg/l). pH of this condensed water is normally between 3 & 4 due to dissolved acid gases (H2S & CO2). If one clean gas well produces 30 to 50 MMSCF/day natural gas, this gas would be normally associated with 30 to 50 bwpd (barrel water per day) of condensed water [of very low salinity (&lt; 200 mg/l)]. A newly drilled/worked-over gas wells produces higher volumes of water (mixture of condensed water, spent acid, filtrate of cement slurries & residual of drilling/completion/work over fluids' filtrate left in surrounding of pay zone hole section). The gas well is considered to be not clean, when it produces higher volumes of water of higher salinity. It is not recommended to flow this well to the gas treatment plant before performing a cleaning operation that could probably last for more than one month of continuous production. Therefore, a newly drilled/worked-over gas well needs to be cleaned thoroughly. i. e., probably up to 1000 MMSCF of natural gas could be needed to be produced ex a newly drilled/worked-over gas well. If this gas would be flared into the atmosphere, the environment would be polluted with ~ 75 tons of CO2 & with about one ton of SO2 (theoretically estimated based on full combustion reactions of natural gas components as per example 3). Example 3: Estimated quantities of CO2 & SO2 polluting the environment due to flaring of about 1000 MMSCF natural into the atmosphere.

Proposed Pilot Trials of Liquid: Gas & Liquid: Liquid Coalescence

Technology for Cleaning Newly Drilled/Worked-Over Gas Wells

To ensure feeding gas plants with uncontaminated effluents as well as to avoid major environmental pollution due flaring produced gases/condensate as well as to minimize the period of depending on costly mobile test separators, the cleaning operations of newly drilled/worked-over gas wells could be performed by utilizing an up to date technology that cleans the natural gas/condensate streams before flowing to the gas plants for further smooth treatment processes without the need to be flared into the atmosphere. This will help also in saving the costs of lengthy MTS operations as well as in preventing wastage of large volumes of natural gases by flaring. Flowing (as per schematic drawing 1) a contaminated gas stream with residuals of drilling/completion fluids from a newly drilled/worked-over well through on-line liquid: gas & liquid: liquid coalescence cartridges after pre separation of bulk liquids & solids could probably help in separating these contaminating waters/solids and allowing the clean natural gas/condensate streams flow to the gas plant for further smooth treatment processes.

Schematic Drawing 1: Proposed Schematic Drawing for Utilizing Of Liquid: Gas & Liquid: Liquid Coalescence Technology for Cleaning Effluents ex Newly Drilled/Worked-Over Gas Wells

The above mentioned coalescence cleaning process could be summarized in 3 stages as follows:STAGE 1: Pre-Separation of Solid Particles from Bulk LiquidDue to the fine pore structure of the coalescence medium, a pre-filter is installed upstream of the coalescence assembly to remove any particulate matter in the stream. The installation of a pre-filter will significantly extend the lifetime of the coalescence medium and reduce particulate concentration in the filter effluent to meet specifications related to content of solids in gas streams flowing to gas treatment plants. Therefore, wherever possible, a preliminary study/test should be carried out to determine the nature, quantity and PSD (Particle Size Distribution) of the solids present in the gas well effluent.STAGE 2: COALESCENCEThe fluid mixture enters the coalescing element and flows inside to outside. This is where small-suspended droplets of the dissimilar fluid come together, or coalesce, as the emulsion moves through the depth of the coalescence medium. Liquid/Gas coalescence elements downstream of the pre-separation column will virtually collect all the undesirable entrained liquids in the flowing gas. This eliminates the problem of hydrocarbons carryover in gas stream. Furthermore the separated condensate from the flowing gas stream can be remixed in the process.STAGE 3: SEPARATIONContaminant free product and large liquid droplets separate in the settling zone of the housing. The separated hydrocarbon (condensate) phase exits through the top drain. The mixture of condensed water with filtrate residuals of drilling/completion fluids exit through the lower drain on the lower section of the housing.CONCLUSIONCost-effectively studied/treated input leads to optimal smooth output. Proactive production chemistry studies aim at preventing technical problems rather than only in appraising/solving them (i. e., prevention is better than treatment). Contaminated gas streams with filtrate residuals of drilling/completion fluids in newly drilled/worked-over wells would cause detrimental effects on the performance of gas treatment plants. During cleaning operations of newly drilled/worked-over gas wells, it is proposed to carry out pilot trials on flowing the natural gas/condensate streams through liquid : gas & liquid : liquid coalescence cartridges before flowing to the gas plants for further smooth treatment processes without the need to be flared into the atmosphere.