Borehole and Invasion Effects of Formate-Based Mud Systems on LWD Density, Neutron, PE, and Gamma-ray Logs
- Paul Andrew Cooper (Sperry-Sun Drilling Services) | James Elmer Galford (Halliburton Energy Services Group) | Grant Goodyear (Halliburton) | Gordon L. Moake (Halliburton Energy Services Group) | Jerome Allen Truax (Halliburton Energy Services Group)
- Document ID
- Society of Petroleum Engineers
- SPE Annual Technical Conference and Exhibition, 30 October-2 November, Denver, Colorado, USA
- Publication Date
- Document Type
- Conference Paper
- 2011. Society of Petroleum Engineers
- 5.6.1 Open hole/cased hole log analysis, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 1.8 Formation Damage, 1.6 Drilling Operations, 1.10 Drilling Equipment, 1.2.3 Rock properties, 2 Well Completion, 1.11 Drilling Fluids and Materials, 5.2 Reservoir Fluid Dynamics, 1.12.2 Logging While Drilling, 6.5.4 Naturally Occurring Radioactive Materials, 2.4.3 Sand/Solids Control, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 4.2.3 Materials and Corrosion, 2.7.1 Completion Fluids
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The popularity of formate-brine drilling muds continues to increase as a result of the benefits in the drilling and completion stages of well construction, as well as the lessened environmental impact when compared to some other muds. Growth in use of these muds has caused operators and service companies to re-examine environmental correction algorithms because of the substantial effects these mud systems can have on nuclear logs. In addition, because formate muds generally contain lower solids than conventional muds, they can be strongly invasive. Thus, the influence of formate muds on LWD logging measurements includes both borehole environmental effects to be corrected and formation invasion effects that affect log interpretation.
A combination of laboratory experiments and Monte Carlo simulations have been used to develop a better understanding of the influences formate muds have on LWD gamma-ray, density, PE, and neutron porosity logs. Several hundred lab experiments and numerical simulations have been conducted in this investigation to fully characterize LWD responses to formate muds for families of sensors comprising two collar sizes used in a large range of hole sizes. The results were used to develop borehole corrections for the nuclear logs.
Formate brines are formulated to achieve a desired fluid density by using a mixture of dissolved compounds: sodium, potassium, and cesium formate. In some instances, the mixture results in a fluid with a substantially lower-than-normal hydrogen index. The mud may also have a larger-than-normal potassium concentration. Because invasion consists of nearly the whole formate mud, correct log evaluation in porous, permeable formations requires interpretation techniques that are beyond the scope of customary borehole corrections. We have taken lab measurements to attempt to quantify the effect of this invasion on LWD tool responses.
Since their introduction in the early 1990s, formate muds have been increasingly used as drilling and completion fluids in challenging well construction environments, especially for HPHT wells. The advantages of formate muds are numerous and well documented (Downs 1993; Howard et al. 1995; Byrne et al. 2002). They have a low solids content, reduced risk of stuck pipe, and exhibit excellent shale stabilization. They cause lower formation damage, and the lower solids content reduces the risk of pore plugging. In addition, these muds have little environmental impact and are less corrosive than conventional completion fluids.
Formates are monovalent-ion organic alkali metal salts. Individually, sodium, potassium, and cesium formates can be used to formulate low-solids drilling muds spanning the range from 8.3 ppg to 19.2 ppg as shown in Fig. 1.
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