Control of Microbiological Activity in Biopolymer-Based Drilling Muds

Abstract

Introduction Deep subsurface terrestrial and marine environments are effectively shielded from the chemical and microbiological influences of surface soils and groundwaters. Physical isolation of the deep subsurface environment has the effect of separating it from normal biogeochemical nutrient recycling. With time, biologically-mediated changes in the deep subsurface environment become insignificant, relative to geothermally- or geochemically-mediated processes. Under such conditions, geologic changes continue until the isolation of the subsurface environment is broken. Abstract Water-based drilling mud formulations routinely include organic polymers as viscosifying and fluid loss control agents. These organic polymers, which are generally of either plant or microbiological origin (hence, "biopolymers"), can be degraded and utilized as nutrients for growth by naturally- occurring oilfield bacteria. This may occur despite the inclusion of biocide in some polymer preparations. Microbial growth in the mud can result in contamination of the well and near-wellbore zone. Fouling, corrosion and reservoir souring may then occur during subsequent operations. If growth in the mud is extensive, significant biopolymer degradation can be expected, with consequent loss of the mud's rheological properties. In these circumstances, additional polymer must be added to the contaminated drilling mud in an attempt to maintain viscosity and prevent excessive fluid loss. As additional biopolymer is added, and biodegradation continues, the cost of the drilling mud increases. This paper addresses the requirement for control of microbiological activity in water-based drilling muds representative of those used in Saudi Arabia. A laboratory investigation of the relative efficacy of alkaline pH (lime) and biocide (glutaraldehyde) treatments in limiting microbiological activity in drilling muds is presented, and minimum treatment levels for absolute control of general aerobic bacteria (GAB) and sulfate-reducing bacteria (SRB) are provided. Application of these findings can be expected to minimize drilling mud biodegradation, and costly post-drilling operational problems such as production tubing and pipeline corrosion. Water injection and oil production occur across the poorly defined physical boundaries separating surface and subsurface environments. They are, by nature, processes which initiate and promote exchange between surface and subsurface environments. Through this process, biogeochemical activity, long curtailed by the physical separation of the two environments can be re-initiated. The result can be renewed or enhanced microbiological activities in both environments. Drilling Activities. Communication between surface and subsurface oilfield environments is initiated through the drilling process. This process requires circulation of fluids (drilling mud) from the surface to the drill bit to carry cuttings out of the borehole. In so doing, chemicals and microbes from the surface are circulated into the deep subsurface, energy-rich oil-bearing strata, and hydrocarbon-laden cuttings are brought into the oxygen-rich, moderate temperature surface environment. Through this process, microbiological activities can be initiated in both surface and subsurface environments which would have been unlikely had exchange between the two environments not occurred. Downhole microbiological activities can cause the following problems:Microbiological corrosion of well tubulars and screens.Biomass plugging in injection wells and in the formation.H2S production deep in the formation, leading to microbial reservoir souring. P. 329^

An Experience in Acidizing Sandstone Reservoirs: A Scientific Approach

“…It is unknown, however, whether these SRB's occur naturally within the system, or were introduced with the drilling mud used across the field. 3 Particulate iron sulfide entering the injection wells would eventually result in formation plugging and subsequent injectivity decline. [4][5][6][7] The sandstone reservoir is highly unconsolidated and heterogeneous as indicated by bulk XRD analysis given in Table 2.…”