Estimation of the equilibrium formation temperature in the presence of bore fluid invasion

Abstract

Bottom hole temperatures (BHTs) measured during drilling operations are thermally disturbed by the drilling process. This paper presents a method, CSMI (Cylindrical Source Model with Invasion of bore mud filtrate), for estimating equilibrium formation temperatures with probability distributions from BHT measurements in the presence of bore fluid invasion. The scheme is based on finite element analysis in conjunction with Markov chain Monte Carlo inversion. The axisymmetric forward model assumes a cylindrical source of finite radius and contrasting thermal parameters, which includes the possibility of invasion (advection) of mud filtrate into the formation. In a synthetic example, it is demonstrated that given bore fluid invasion and a low and high temperature of the bore mud and formation, respectively, the equilibrium formation temperature and the uncertainty hereon is underestimated by correction schemes based on purely conductive models. The influence of the borehole radius and fluid invasion on the temperature measured at the borehole axis attenuates over time. It is further demonstrated that the invasion radius and the matrix thermal conductivity cannot be estimated simultaneously with the CSMI scheme. The analysis of five BHT records measured onshore Denmark, for which the equilibrium formation temperature is known, shows that CSMI temperatures based on single datum records are highly uncertain because of a strong negative coupling between the temperature of the mud filtrate and the equilibrium formation temperature. For records with multiple temperature measurements, the CSMI scheme matches statistically the measured equilibrium formation temperatures. It is further shown that additional negative bias is added to Horner plot temperatures if bore fluid invasion has occurred. Allowing for bore fluid invasion in addition to a borehole of finite radius and contrasting thermal parameters, increases temperature estimates by 5 per cent (4-7 per cent) on average given that invasion is indicated and that the bore fluid is significantly colder than the formation. Indications of bore fluid invasion from CSMI analysis are confirmed in two examples by analysing corresponding caliper logs. © 2012 The Authors Geophysical Journal International © 2012 RAS.