Applying a Geologist’s Tools to Decipher Scaling History: Integration of Mineralogy, Morphology, and Polymorphic Differentiation

Commonly, only very limited amounts (10-200 g) of sample can be retrieved by bailers or on the filter screens at well heads and the usual evaluation is simply XRD, XRF and occasionally, SEM analysis to identify basic mineralogy. Unfortunately, these very small and poorly differentiated samples are the only evidence production engineers have to evaluate significant issues with completion and drilling chemistry. But, a geologist’s eye, and polymorph chemistry can use these small samples to dictate a story. In a recent study, meticulous sample preparation and visual sample evaluation, combined with XRD, thin section petrography, crystal morphology, and evaluation of polymorphic and formation water chemistry have been intertwined to evaluate scaling history in several wells in a high temperature, high pressure gas condensate field. The study was able to identify various polymorphs of CaCO3, (including Mg-calcite and aragonite), and assign them to quiescent vs. dynamic precipitation regimes, through the assessment of morphological features (including botryoidal, dendritic and flos ferri morphologies of aragonite). In addition, multiple stages of barium sulfate (BaSO4) and sulfate/carbonate co-precipitation were identified. An additional fecarbonate phase, siderite (FeCO3), was identified. Using thin section petrology, mineralogy, and a thorough review of drilling and completion operations, the unique scale formation history of these wells was traced, and issues were identified with long-term post-completion shut-ins while waiting for tie-in of manifolds to pipelines. Issues were identified with CO3 -2 pH buffers in formate based completion and drilling fluid interactions with high calcium formation brine, which migrates to the well bore by ionic diffusion and counter-current imbibition. The identification of this previously un-suspected damage mechanism has led to modification in completion fluid formulations to eliminate future events, and improve post drilling PI on wells drilled in the gas fields in question. It highlighted the requirement for a scale control program which is being developed to be concurrent with new well drilling and completion. Finally, the study highlights the need to evaluate fully the components of drilling, completion and stimulation fluids, both individually, and in combination–To be sure to fully assess the risk of formation damage, even when using a welltested and proven industry fluid. The phrase “doveryai, no proveryai” (“trust, but verify”) takes on a new meaning (with due respects to US Pres. R. Regan).