X-ray densitometry of drilling fluids at the rig standpipe

Abstract

Fluids used during well construction operations serve as a primary barrier for well control, with the fluid density generating sufficient hydrostatic pressure in the fluid column to control downhole formation fluids and gas. Real-time density measurement at the well inlet, where high pressures of many thousands of psi can occur, is highly desirable in order to maintain well control. This improved characterization of the well bore pressure profile allows one to drill wells with narrow drilling margins without non-productive time from well control events, resulting in safer and more cost-effective well construction operations. Gamma ray densitometers are suitable measuring devices for high-pressure density measurement. Typical flow conditions encountered at the inlet, such as a fully filled pipe and a homogeneously mixed mud, are very conducive to their use. In-spite of their high measurement accuracy and relatively fast response times, however, these meters have seen limited adoption in field. This is primarily because the meters come with safety concerns due to the radioactive source required for gamma ray generation. Additionally, the source has to be tracked throughout its life and properly handled during storage, transportation, installation and disposal. Given these concerns and challenges, a majority of oil and gas operating and service companies use manual density measurements at the mud pits using a pressurized mud balance. To help change this rather antiquated and infrequent practice that relies on manual labor, this paper evaluates the feasibility of using a non-radioactive, X-ray based densitometer as an alternative to the gamma ray meter. X-ray densitometry works on the same proven principles as gamma ray densitometry, without the latter's drawbacks. Two X-ray methods, an empirical method and a model-based method (inspired by a technique used in the medical field known as single X-ray absorptiometry) are presented here. Real-time density measurements on the high pressure line with an accuracy of 99% or better and a measurement frequency of 1 Hz are feasible using these methods.