ARPO ENI S.p.A. Agip Division IDENTIFICATION CODE STAP-P-1-M-6100 PAGE 11 OF 230 REVISION 0 Equations used by ENI Agip division for fracture gradient calculation, (when overburden gradients and pore pressure gradients have been defined), are listed below: Terzaghi equation (commonly used):
2??Gf?? Gp???(Gov??? Gp)
1?????When the formation is deeply invaded with water:
Gf?? Gp?? 2? (Gov??? Gp)
When the formation is plastic:
Gf?? Gov
where:
Gf Gov Gp v ??
= = = = =
Fracture pressure Overburden gradient Formation pressure Poissions modulus
0.25 for clean sands, sandstone and carbonate rocks down to medium
depth
0.28 for sands with shale, sandstone and carbonate rocks at great depth.
when Poisson?s modulus may have the following values:
??
=
2.2.
OVERPRESSURE EVALUATION
There are three methods of qualitative and quantitative assessment of overpressure:
a) b) c)
Methods before drilling Methods while drilling Methods after drilling.
ARPO ENI S.p.A. Agip Division 2.2.1.
Methods Before Drilling
IDENTIFICATION CODE STAP-P-1-M-6100 PAGE 12 OF 230 REVISION 0 Gradients prediction is based, on the most part, analysis and processing of seismic data and data obtained from potential reference wells. This includes:
Drilling Records
These can be used in determining hole problems, abnormal pressures, lost circulation zones, required mud weights and properties, etc.
Wireline Logs
These can provide useful geological information such as
lithology, formations tops, bed thicknesses, dips, faults, wash out, lost circulation zones, formation fluid content and formation fluid pressure (pore pressure).
Seismic Surveys
Provides two of the most important applications of seismic data in; the detection of formations characterised by abnormal pressures and; in the forecasting of probable pressure gradient. The data from seismic surveys are analysed and interpreted to evaluate transit times and propagation velocity for each interval in the formation. Since overpressurised zones have a porosity higher than normal, it is reflected in a travel time increase.
It is obvious that if the drilling is explorative and is the first well in a specific area, the seismic data analysis may be the sole source of information available.
The prediction of the gradients is essential for planning the well and must be included in the drilling programme. This initial drilling phase may be able to detect zones of potential risk but cannot guarantee against the potential presence and magnitude of abnormal pressures and, hence caution must be exercised.
2.2.2.
Methods While Drilling
Given all the predictive methods available, successful drilling still depends on the effectiveness of the methods adopted and on the way they are used in combination. Although most of these methods do not provide the actual overpressure picture, they do signal the presence of an abnormal conditions due to the existence of an abnormally
behaving zone. Such methods, therefore, provide a warning that a more careful and diligent observation must be maintained on the well.
The most critical situation occurs when a well with normal gradient penetrates a high pressure zone without any indications caused by faulting or outcropping at a higher
elevation. However, when abnormal pressure occurs as a result of compaction only, many of the following real time indicators appears before a serious problem develops.
ARPO ENI S.p.A. Agip Division 2.2.3.
Real Time Indicators Penetration Rate
IDENTIFICATION CODE STAP-P-1-M-6100 PAGE 13 OF 230 REVISION 0
While drilling in normal pressured shales of a well, there will be a uniform decrease in the drilling rate due to the increase in shale density. When abnormal pressure is encountered, the density of the shale is decreased with a resultant increase in porosity. Therefore, the drilling rate will gradually increase as the bit enters an abnormal pressured shale. The corrected ?d? exponent and Eni-Agip Sigmalog eliminate the effects of drilling parameter variations and give a representative measure of formation drillability.
The TDC Engineer is responsible for continuous monitoring and shall immediately report to the Company Drilling and Completion Supervisor, if any change occurs.
A copy of corrected the ?d? exponent or Agip Sigmalog shall be sent on daily basis to the Company?s Shore Base Drilling Office by telefax for further checking.
Drilling Break
A drilling break is defined as a rapid increase in penetration rate after a relatively long interval of slow drilling.
Any time a drilling break is noticed, drilling shall be suspended and a flow check carried out. If there is any lingering doubt, the hole will be circulated out until bottoms up.
Torque
Torque sometimes increases when an abnormally pressured shale section is penetrated due to the swelling of plastic clay causing a decrease in hole diameter and/or accumulation of large cuttings around the bit and the stabilisers.
Also torque is not easy to interpret in view of many
phenomena which can affect it (hole geometry, deviation, bottom hole assembly, etc.), it must be thought as the
second-order parameter for diagnosing abnormal pressure.
Tight Hole During Connections
Tight hole when making connections can indicate that an abnormal pressured shale is being penetrated with low mud weight. When this occurs it is confirmed when the hole must be reamed several times before a connection can be made. When making up connections, cavings may settle preventing the bit returning to bottom.
Wall instability, in an area of abnormal pressure, may cause sloughing. It should be noted that fill may be due to other causes, such as wall instability through geomechanical reasons (fracture zones), inefficient well cleaning by the
drilling mud, rheological properties of mud insufficient to keep cuttings in suspension, etc.
Hole Fill
ARPO ENI S.p.A. Agip Division MWD
IDENTIFICATION CODE STAP-P-1-M-6100 PAGE 14 OF 230 REVISION 0 In addition to directional drilling data, MWD can provide a wide range of bottom hole drilling parameters and formation evaluation, e.g.: bottomhole weight on bit, torque at bit,
gamma ray, mud and formation resistivity, mud pressure and mud temperature.
If the true weight and torque at the bit are known, the drilling rate can be normalised with more accuracy by producing a more accurate ?d? exponent and Agip Sigmalog.
Formation resistivity is plotted and interpreted for pressure development. It should also be noted that differential resistivity between the mud in the drill pipe and in the annular space may be considered as a kick indicator.
2.2.4.
Bottomhole mud temperature can also be an indicator of overpressure as discussed below.
Indicators Depending on Lag Time Mud Gas
The monitoring and interpretation of gas data are fundamental to detecting abnormally pressured zones.
? Background gas is the gas released by the formation while drilling. It usually is a low but steady level of gas in the mud which may be interrupted by higher levels resulting from the drilling of a hydrocarbon bearing zone or from trips and connections.
? An increase in the level of background gas, from that
previously found in overlying normally compacted shales, often occurs when drilling undercompacted formations. ? Gas shows can occur when porous, permeable formations containing gas are penetrated. Monitoring the form and the volume of gas shows will make it easier to detect a state of negative differential pressure.
? Trip gas may be an indication of well underbalance. The equivalent density applied to the formation with pumps off (static) is lower than the equivalent circulating density (dynamic) and when the well is close to balance point, the drop in pressure while static may allow gas to flow from the formation into the well. The quantity of gas observed at the surface when circulation is resumed, however will depend on several factors, e.g., differential pressure, formation permeability, drill pipe pulling speed, swabbing. Failure to fill the hole on trips may also cause an increase in trip gas. ? Connection gas may be an indication of well imbalance (see above).
? The progressive changes, or trend, in connection gases is an important aid to evaluate differential pressure. When an undercompacted zone of uniform shale is drilled without increasing the mud weight, the amount of connection gas will almost always increase.
ARPO ENI S.p.A. Agip Division Mud Temperature
IDENTIFICATION CODE STAP-P-1-M-6100 PAGE 15 OF 230 REVISION 0 Measurement of mud temperature can also be used to detect undercompacted zones and, under ideal conditions, or to anticipate their approach. This is because temperature
gradients observed in undercompacted series are, in general, abnormally high compared with overlying normally pressured sequences.
Accurate interpretation of these data is very difficult, due to a number of variables which frequently mask changes in geothermal gradient:
? Inflow temperature, which is dependent on the amount of cooling at surface.
? Flow rate, which affects the speed at which the mud, and the calories it contains, returns up the annulus. ? Thermophysical properties of the mud. ? Heating effects at the bit face.
? Heat exchange in the marine riser between the mud and the sea.
? Halts in drilling and/or circulation.
? Surface operations such as transfer of mud between pits, etc.
? Lithology: the lithological sequence may provide an overall indication of the possible existence of abnormal pressure. The presence of seals, drains or thick clay sequences is a determining factor in this analysis.
? Shale density: is based on the principle that bulk density in an undercompacted zone does not follow the trend of the normally compacted overlying clays and shales. The validity of the density obtained depends on the clay composition (the presence of accessory heavy minerals can greatly change the density), the depth lagging (which can make cutting selection difficult), the mud type (reactive muds have an adverse effect on measurement quality) and clay consolidation (difficult to measure on wellsite the density of clays not sufficiently consolidated).
? Shale factor: undercompacted clays which have been unable to dehydrate often have an unusually high
proportion of smectite and an abnormally high shale factor. However, the initial proportions of the clay minerals in the deposit can mask changes in shale factor and give a false alarm.
? Shape, size and volume of cuttings: the amount of shale
cuttings will usually increase, along with a change in shape, when an abnormal pressure zone is penetrated.
Cutting Analysis