Gas logging and hydrodynamic logging

Gas logging and hydrodynamic logging
MUD LOGGING is a well logging service to provide the monitor well site activities and to analyze the drilling cuttings (core, for instance) for lithology identification and hydrocarbon evaluation. The resulting plots of those activities and cutting analysis versus depth is termed a MUD LOG.

Basic mud logging involves:
O Lag time determination
o Depth and ROP determination
o Cuttings sampling and lithological description
o Gas sampling analysis
o Oil shows evaluation
o Drilling parameters monitoring
Gas Liberated by Drilling
Liberated gas is gas that enters the mud stream as a direct consequence of drilling. The gas is mechanically liberated by the bit into the drilling fluid as the bit penetrates the formation.
Recycled gas which is pumped back into the borehole by circulating mud to appear a second time at the surface.
Produced gas enters the annulus out of rock formations exposed in the wellbore from a specific zone in response to a formation pressure which exceeds the opposing effective hydrostatic pressure.
Contaminant gas is gas artificially introduced in the mud stream from a source other than the rock formations such as using an oilbase mud or adding diesel to a water- base system.
Influx and Flushing
Influx occurs when hydrostatic pressure is low enough to permit formation fluids to enter the well bore; flushing occurs under reciprocal conditions.The rate of influx or flushing across the wellbore interface depends principally on pressure and permeability.
Sample Lag
Lag time for both cuttings and fluids is generally determined by a two-step process:
1. The addition of a tracer into the down hole side of the mud circulating system
2. The detection of the tracer when it returns to surface on the up hole side
The most commonly used tracer is calcium carbide, a manmade crystalline solid that reacts spontaneously with water to form acetylene gas:
CaC2 + 2H2O Ca(OH)2 + C2H2 Calcium Carbide + Water Calcium Hydroxide + Acetylene Gas
Phase and Volume Changes During Up hole Travel
In traveling from hole bottom to surface, rock debris, natural fluids, and circulating mud experience great changes in temperature and pressure conditions
• Mud temperature may decrease by as much as three degrees centigrade for each hundred meters of upward travel
• Pressure changes during up hole travel are also extreme. Hydrostatic pressure in the borehole is a function of the mean density and vertical height of the fluid column.
• Careful monitoring is necessary to discriminate between gas influx or well kick due to underbalanced drilling and the arrival of a large volume of gas-aerated mud resulting from penetration of a gas-bearing zone.
Gas Trap Sampler
An internal impeller draws mud into the trap through an upstream port, agitates it to lower its viscosity and free the entrained gas, and discharges it through a downstream port to the mudlogging unit. Here the sample passes through filtration and further drying steps and is routed and metered to various gas detectors and analysers.
Gas Detection and Measurement
1.Catalytic Combustion Detector (CCD) – obsolete In this device, the filtered and dried gas sample is passed continuously, at a controlled flow rate, through a combustion chamber containing a heated platinum filament.
2. Flame Ionization Detector (FID) Detection process starts with ionization of a sample into charged hydrocarbon in a high temperature hydrogen flame.
Gas Compound Separation
Gas chromatography is the principal separation method used in mud logging Gas Chromatography (GC) Chromatography is a separation method in which a complex mixture is passed through a medium that retards individual compound types at different rates. In practice, mud-logging gas chromatography encompasses four sequential steps: sample collection, sample injection, chromatographic separation, and compound detection. These are carried out with the sample flow loop, batch sampler, separation columns, and detector.
Identification of Heavy Oil (Tar Mats) Deposits
Tar mat (asphaltic) interval near oil–water contact and influences reservoir production and the aquifer support.
GasData: Practical Applications The two broad uses of mud gas data in formation logging are:
1. Correlation: recognition of similarities with other logs and wells, the total combustible gas plot combined with the ROP or lithology curve can often be used visually for direct formation correlation with mud gas and wireline logs from adjoining wells.
2. Evaluation: recognition of anomalies in the well being drilled. Gas Ratio Analysis When using either plot, the following corrections must be made:
Removal of all contamination gas readings, such as diesel, trip gas, connection gas, recycled gas
Correction for background gas. The relative concentrations must be read above background gas
More than one reading must be done to have any interpretative value
Hydrodynamic logging