The popularity of formate-brine mud systems has increased significantly over the past several years, in large part because they benefit the drilling and completion stages of well construction. The advent of these muds has caused both operators and service companies to re-examine environmental correction algorithms because of the substantial effects the mud systems can have on basic nuclear logs. Moreover, since formate muds generally contain lower solids compared to conventional muds, they tend to be strongly invasive. Thus, the influence of formate muds on logging measurements includes both borehole effects and formation-interpretation effects arising from mud-filtrate invasion.
A combination of laboratory experiments and Monte Carlo simulations have been used to develop a better understanding of the influences formate muds have on wireline gamma-ray, density, and thermal-neutron porosity logs. More than 220 laboratory experiments and 330 numerical simulations were made in this investigation. The results were used to develop robust borehole corrections for thermal-neutron porosity as well as for total- and spectral-gamma-ray logs. An additional 40 laboratory measurements were also performed to check for unique formate-mud effects on spectral density measurements that may require special treatment.
Formate brines are formulated to achieve a desired fluid density by using one or more compounds, e.g., sodium, potassium, or cesium formate. In some instances, the mixture results in a fluid with a substantially lower-than-normal hydrogen index and/or a larger-than-normal potassium concentration. As a consequence, the proper evaluation of nuclear logs in the presence of formate-mud invasion into porous, permeable formations requires interpretation techniques that are beyond the scope of customary borehole corrections. This paper introduces a simplistic model, which combines formation parameters with nuclear measurements to estimate the range of additional effects associated with formate-mud invasion.