MUD LOGGING GROWING CONTRIBUTION TO GEOCHEMISTRY
M.A. Chiaramonte
GEOLOG, Italy
Mud logging (ML) is an historical activity, run at well site since the late ’30 and its main
tasks have been for many years to record depth, to describe lithology, to collect cutting
samples and, above all, to prevent kicks by monitoring gas entering in the hole. Analyses of
gas, desorbed from the mud at well head, have become one of the main task of ML,
increasing, progressively through time, type and number of measured hydrocarbon
molecules. Nowadays ML is able to continuously analyze light hydrocarbons (LHs) in the
range C1-C8, including a large part of different isomers. ML has gradually entered in this
way in the domain of geochemistry and of reservoir evaluation.
Hydrocarbon extraction process from mud is a key point for ML quality and reliability, its
standardization and its efficiency evaluation have been for a long time a subject of research.
If hydrocarbon recovery from mud is reliable and quantitative, an important sample type is
made available for geochemical investigations: LHs naturally released by cuttings, as
consequence of depressurization during travelling from bottom hole to surface. Many
samples of this type can be taken and analyzed, thanks to a very short analytical turnaround
time, during ML services, allowing definition of a LH composition profile, along the whole
source rock or reservoir section. Quantitative recovery of LHs from cuttings is a very
difficult exercise, because cuttings, sampled at shale shakers, have already released the
greatest part of them, during transport to surface. The small portion of LHs, still present in
cuttings, can be easily lost during any type of sample treatment. A possible methodology to
recover LHs from cuttings (hydrogen stripping) was given by Schaefer (Schaefer et. al.,
1978).
Hydrocarbon extraction from mud has been largely improved through optimization of
geometry of extraction equipment, of its temperature, of mud inlet flow, of stirring conditions
and of many other parameters. Constant temperature, pressure, volume and flow are keys to
achieve reliable and standardized data. CFD (Computational Fluid Dynamics) numerical
simulations greatly helped in defining optimal conditions, drastically reducing the time
needed for experimental work and simplifying it. Extraction efficiency, obtained in this way,
can be accurately quantified by measuring the total gas originally dissolved in the mud. This
goal can be achieved by putting mud in a sealed vessel and measuring gas concentrations at
the equilibrium in the head space. The total gas can be then obtained by calculating the gas
in the mud phase by means of partition coefficients (Doninelli et al.,2015). In this way, ML
can easily provide many LH reliable data, expanding the use of this class of hydrocarbons,
limited for many years mainly to well preserved oil samples. The ratio between different type
of LH molecules, obtained in this way, are comparable to those obtained from PVT samples.
Many papers (just to mention some: Thomson,1979, Odden et. al, 1998, Mango, 1997)
suggest how to use and interpret these data. LH data can be used for oil-source rock
correlations, but they can also give detailed information about organic facies in the source
rock and about fluid distribution and maturity within the reservoir.
On extracted gas, carbon stable isotopes analyses can also be performed in the range C1-C3
at well site with a short analytical turnaround time (8 minutes for C1-C3 analyses, 3
minutes for C1-C2) making possible a detailed isotopic profile of gas in the well (see fig. 1).
Samples of gas extracted from mud are usually also taken for lab analyses, but the high
28th International Meeting on Organic Geochemistry
17 – 22 September 2017, Florence, Italy
resolution isotopic profile can provide unique information and it is producible only at well
site, by using a great number of samples,. As matter of fact this approach has put in evidence
non homogeneous isotopic profiles in some reservoirs, difficult to highlight with lab analyses
on spot samples but of great interest for their consequence on operational decisions. In
addition to this, very interesting results were obtained by comparison of isotopic value of
mud gas and of head space gas, released by cuttings sealed in vials. The possible impact of
reservoir permeability on isotopic differences measured between mud gas and head space
samples at the same depth has been already discussed by some papers ( Xinyu et al., 2011,
Madren, 2011).
Figure 1 Example of isotopic composition profile for ethane in a reservoir. Light blue dots,
show results of isotopic analyses at well site, dark blue and yellow dots are the extraordinary
numerous samples, analysed in two different labs, to validate wellsite measurements.
References
Doninelli D., Rizzola J, Gronchi P., Da Rù L., 2015, Advances in the Hydrocarbon GasLiquid Equilibrium Understanding in Water and Oil-Based Drilling Fluids, Offshore
Mediterranean Conference 2015, OMC-2015 -262
Odden, W., Patience, R.L., Van Graas, G.W., 1998, Application of light hydrocarbons (C4C13) to oil-source rock correlations: a study of the light hydrocarbon compositions of
source rocks and test fluids from Offshore Mid-Norway, Org. Geochem, 28, 823-847
Madren, J.D., 2011, Isotopic Fractionantion of Methane in Shale as a Predictor of Matrix
Deliverability, AAPG Search and Discovery Article #90134, Hedberg Conference Natural
Gas Geochemistry
Mango F.D. , (1997), The light hydrocarbons in petroleum : a critical review, Organic
Geochemistry, 7-8, 417-440
Schaefer, R.G., Weiner, B., Leythauser, D.,1978, Determination of sub nanonogram per gram
quantities of light hydrocarbons (C2-C9) in rock samples by hydrogen stripping in the flow
system of a capillary gas chromatograph. Anal Chem. 50 , 1948-1854
Thompson, K.F. M., 1979, Light hydrocarbons in subsurface sediments, Geochim. et
Cosmochim. Acta, 43, 657-672
Xinyu, X. Tang, Y. , 2012, Isotope fractionation of methane during natural gas flow with
coupled diffusion and adsorption/desorption, Geochim. et Cosmochim. Acta, 77, 489–503
28th International Meeting on Organic Geochemistry
17 – 22 September 2017, Florence, Italy