Geological Indicators of Increasing Pressure
•Size and shape of cuttings cont
–These cuttings should not be confused with even larger, block shaped cuttings, which are rectangular.
–These block-shaped cuttings do not originate from the bottom of the well.
–They are formed by improper drillstring and bottom-hole assembly mechanics or existing fracturing.
•Sloughing shale and abnormal hole fill-up
–Sloughing shale and abnormal hole fill-up are indications of increasing formation pressure.
–As the transition zone is penetrated, the pore pressure within the shale will increase.
–Shales have relatively low permeability, but in a transition zone, shale porosity will increase.
•Sloughing shale and abnormal hole fill-up Cont.
–If this overpressure in the shale is not offset by increasing the hydrostatic pressure of the mud, the shale will collapse or slough into the annulus.
–This can cause enlarged holes through transition zones and fill on bottom during connections and trips.
•Bulk Density
–During normal shale compaction, water is squeezed out of the shale as the overburden pressure increases.
–Shale porosity decreases and density increases with depth.
–If normal compaction is interrupted by the formation of a seal, the formation water cannot be squeezed out of the shale.
•Bulk Density Cont.
–When this occurs, the fluid supports part of the overburden load and will have higher-than-normal pressure.
–Since fluids remain in the shale, the shales have a higher-than-normal porosity and lower-than-normal density.
•Bulk Density Cont.
–If shale densities are checked and plotted at regular intervals during drilling, a normal compaction trend is established for the predominant formation being drilled.
–When a seal is penetrated, the formation density will increase rapidly, followed by decreased density as the over compacted pressure seal and transition zone are drilled.
•Gas
–Gas is an indication of underbalanced formation pressure.
–When drilling is underway, most well-logging companies measure and record the gases entrained in the circulating fluid. It is helpful to classify this gas into one of three different categories:
•Background Gas
•Connection Gas
•Trip Gas
•Background gas
–This is the total gas entrained in the mud.
–The background gas which comes from the cuttings as the hole is being drilled is not an indication of increasing pressure and should not be compensated for with higher mud weight.
–Background gas from cuttings should always be circulated bottoms-up.
•Background gas cont
–A continued increase in background gas indicates a higher formation porosity and/or a higher hydrocarbon saturation in the available pore space.
–If lithology and ROP are given due consideration, an increase in background gas would indicate drilling into a transition zone.
•Connection gas
–the amount of gas in excess of the background gas.
–This is the increase in gas readings caused by the swabbing action of drillstring movement while a pipe connection is made.
–Pulling of the drillstring causes the effective bottom-hole pressure to be less than the hydrostatic pressure of the mud column.
–Such a reduction in hydrostatic pressure could lead to formation fluids feeding into the hole.
–A small but constant amount of connection gas is an indicator that the formation pressure is slightly less than the hydrostatic pressure, whereas a continuous increase of gas at each connection would indicate an increase in formation pressure.
–This is an excellent tool for detection of abnormal pressures when used in conjunction with background gas.
•Trip gas.
–This is the increase in gas associated with pulling the drillstring out of the hole.
–Trip gas is recorded when bottoms-up is being circulated out after a trip.
–The time period during which trip gas is being recorded gives some idea about the amount and the migration of gases in the
–annulus.
–This parameter is used in the same manner as connection gas, but is not as useful due to the long interval between trips.
–In some instances, a short trip will be made (10 to 20 stands) for the purpose of determining changes in pore pressure and changes in bottom-hole conditions.
•Gas-cut mud
–Gas-cut mud is the reduction in mud weight due to gas entrainment.
–Gas-cut mud is checked at the flow line, where the fluid will contain the maximum amount of gas.
–The use of gas-removal equipment, as well as surface retention time, will normally remove most or all of the gas from the mud.
–A continued reduction in mud weight due to gas is an indication of increasing gas content in the formations and the potential of increasing pore pressures
•Chloride ion
–Dissolved solids in the formation water are often correlated to total chloride concentration —or salinity, as it is commonly called.
–The salinity of water found in shale is known to increase with depth in a normally compacted sedimentary basin, but shows a decrease in a transition zone.
–In normally compacted formations, the salinity of water found in sandstone is known to follow the same trend, but at much higher concentrations than those found in shale.
–In a transition zone, the salinity of water in sands approaches that of water in the shales.
–The change in the salinity of the mud filtrate is not used for detecting abnormal pressures because it is affected by numerous variables and could give an erroneous indication of a transition zone.
•Flow-line temperature
–Increasing flow-line temperature is an excellent indicator of a transition zone.
–Since certain other variables affect flowlinetemperature, it is necessary to usean end-to-end plot.
•An end-to-end plot is constructed by identifying changes in flow-line temperature caused by a change in the variables, rather than a change in formation pressure
Flow-line temperature cont.–A normal trend can be established and departures from the normal trend can be readily recognized.–An end-to-end plot will produce a curve as shown in the Figure
–At about 150 to 300 ft above the seal, a marked decrease in flow-line temperature will be noted (Point A in the Figure).
–Usually, this decrease is 18 to 20°.
–After the seal is drilled, a very rapid increase in temperature will occur —to perhaps as much as 30 to 35°from the time the seal is drilled until a porous zone is encountered.
–Changes in flow-line temperature cannot be used to estimate formation pressures directly, due to flow-line temperature variables and because each geographic area has a different temperature gradient.
–Changes in flow-line temperature are a qualitative indication that a change in pressure may be occurring.
–These cuttings should not be confused with even larger, block shaped cuttings, which are rectangular.
–These block-shaped cuttings do not originate from the bottom of the well.
–They are formed by improper drillstring and bottom-hole assembly mechanics or existing fracturing.
•Sloughing shale and abnormal hole fill-up
–Sloughing shale and abnormal hole fill-up are indications of increasing formation pressure.
–As the transition zone is penetrated, the pore pressure within the shale will increase.
–Shales have relatively low permeability, but in a transition zone, shale porosity will increase.
•Sloughing shale and abnormal hole fill-up Cont.
–If this overpressure in the shale is not offset by increasing the hydrostatic pressure of the mud, the shale will collapse or slough into the annulus.
–This can cause enlarged holes through transition zones and fill on bottom during connections and trips.
•Bulk Density
–During normal shale compaction, water is squeezed out of the shale as the overburden pressure increases.
–Shale porosity decreases and density increases with depth.
–If normal compaction is interrupted by the formation of a seal, the formation water cannot be squeezed out of the shale.
•Bulk Density Cont.
–When this occurs, the fluid supports part of the overburden load and will have higher-than-normal pressure.
–Since fluids remain in the shale, the shales have a higher-than-normal porosity and lower-than-normal density.
•Bulk Density Cont.
–If shale densities are checked and plotted at regular intervals during drilling, a normal compaction trend is established for the predominant formation being drilled.
–When a seal is penetrated, the formation density will increase rapidly, followed by decreased density as the over compacted pressure seal and transition zone are drilled.
•Gas
–Gas is an indication of underbalanced formation pressure.
–When drilling is underway, most well-logging companies measure and record the gases entrained in the circulating fluid. It is helpful to classify this gas into one of three different categories:
•Background Gas
•Connection Gas
•Trip Gas
•Background gas
–This is the total gas entrained in the mud.
–The background gas which comes from the cuttings as the hole is being drilled is not an indication of increasing pressure and should not be compensated for with higher mud weight.
–Background gas from cuttings should always be circulated bottoms-up.
•Background gas cont
–A continued increase in background gas indicates a higher formation porosity and/or a higher hydrocarbon saturation in the available pore space.
–If lithology and ROP are given due consideration, an increase in background gas would indicate drilling into a transition zone.
•Connection gas
–the amount of gas in excess of the background gas.
–This is the increase in gas readings caused by the swabbing action of drillstring movement while a pipe connection is made.
–Pulling of the drillstring causes the effective bottom-hole pressure to be less than the hydrostatic pressure of the mud column.
–Such a reduction in hydrostatic pressure could lead to formation fluids feeding into the hole.
–A small but constant amount of connection gas is an indicator that the formation pressure is slightly less than the hydrostatic pressure, whereas a continuous increase of gas at each connection would indicate an increase in formation pressure.
–This is an excellent tool for detection of abnormal pressures when used in conjunction with background gas.
•Trip gas.
–This is the increase in gas associated with pulling the drillstring out of the hole.
–Trip gas is recorded when bottoms-up is being circulated out after a trip.
–The time period during which trip gas is being recorded gives some idea about the amount and the migration of gases in the
–annulus.
–This parameter is used in the same manner as connection gas, but is not as useful due to the long interval between trips.
–In some instances, a short trip will be made (10 to 20 stands) for the purpose of determining changes in pore pressure and changes in bottom-hole conditions.
•Gas-cut mud
–Gas-cut mud is the reduction in mud weight due to gas entrainment.
–Gas-cut mud is checked at the flow line, where the fluid will contain the maximum amount of gas.
–The use of gas-removal equipment, as well as surface retention time, will normally remove most or all of the gas from the mud.
–A continued reduction in mud weight due to gas is an indication of increasing gas content in the formations and the potential of increasing pore pressures
•Chloride ion
–Dissolved solids in the formation water are often correlated to total chloride concentration —or salinity, as it is commonly called.
–The salinity of water found in shale is known to increase with depth in a normally compacted sedimentary basin, but shows a decrease in a transition zone.
–In normally compacted formations, the salinity of water found in sandstone is known to follow the same trend, but at much higher concentrations than those found in shale.
–In a transition zone, the salinity of water in sands approaches that of water in the shales.
–The change in the salinity of the mud filtrate is not used for detecting abnormal pressures because it is affected by numerous variables and could give an erroneous indication of a transition zone.
•Flow-line temperature
–Increasing flow-line temperature is an excellent indicator of a transition zone.
–Since certain other variables affect flowlinetemperature, it is necessary to usean end-to-end plot.
•An end-to-end plot is constructed by identifying changes in flow-line temperature caused by a change in the variables, rather than a change in formation pressure
Flow-line temperature cont.–A normal trend can be established and departures from the normal trend can be readily recognized.–An end-to-end plot will produce a curve as shown in the Figure
–At about 150 to 300 ft above the seal, a marked decrease in flow-line temperature will be noted (Point A in the Figure).–Usually, this decrease is 18 to 20°.
–After the seal is drilled, a very rapid increase in temperature will occur —to perhaps as much as 30 to 35°from the time the seal is drilled until a porous zone is encountered.
–Changes in flow-line temperature cannot be used to estimate formation pressures directly, due to flow-line temperature variables and because each geographic area has a different temperature gradient.
–Changes in flow-line temperature are a qualitative indication that a change in pressure may be occurring.
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