BOREHOLE AND FORMATION INVASION EFFECTS OFFORMATE-BASED MUD SYSTEMS ON NUCLEAR LOGS

摘要

The popularity of formate-brine mud systems hasincreased significantly over the past several years, inlarge part because they benefit the drilling andcompletion stages of well construction. The advent ofthese muds has caused both operators and servicecompanies to re-examine environmental correctionalgorithms because of the substantial effects the mudsystems can have on nuclear logs. Moreover, becauseformate muds generally contain lower solids comparedto conventional muds, they can be strongly invasive.Thus, the influence of formate muds on loggingmeasurements includes both borehole effects andformation-interpretation effects arising from mudfiltrateinvasion.A combination of laboratory experiments and MonteCarlo simulations have been used to develop a betterunderstanding of the influences formate muds have onwireline gamma-ray, density, and thermal-neutronporosity logs. More than 120 laboratory experimentsand 340 numerical simulations were made in thisinvestigation. The results were used to develop robustborehole corrections for thermal-neutron porosity aswell as total- and spectral-gamma-ray logs. Anadditional 40 laboratory measurements were alsoperformed to check for unique formate-mud effects onspectral density measurements that may require specialtreatment.Formate brines are formulated to achieve a desired fluiddensity by using one or more compounds, e.g., sodium,potassium, or cesium formate. In some instances, themixture results in a fluid with a substantially lowerthan-normal hydrogen index and/or a larger-thannormalpotassium concentration. As a consequence, theproper evaluation of nuclear logs in the presence offormate-mud invasion into porous, permeableformations requires interpretation techniques that arebeyond the scope of customary borehole corrections.This paper introduces a simplistic model whichcombines formation parameters with nuclear loggingmeasurements to estimate the range of additionaleffects associated with formate-filtrate invasion.