Drilling in high pressure

Friday, September 2, 2011

How does high pressure occur in the subsurface, what are the hazards to drilling through it, and how can you be best prepared, and does it help to do a pressure study of the entire region? Richard Swarbrick, Stephen O'Connor and Richard Lahann of Ikon Geopressure explain

The BP Macondo oil spill in the Gulf of Mexico highlighted the technical challenge of drilling for oil in deep water.

The high pressures there added complexity to the control incident and the high volumes of fluids which blew out to the seabed.

The rate of pressure increases in the subsurface can be very high. In Brunei, for example, an increase in overpressure of 220 bar (3200psi) has been reported across a shale only 17m (55 feet) thick. There have been reports of a greater than 230bar (3300psi) increase in overpressure over 60m vertical section at the base of the sand-rich facies in the Gulf of Mexico.

The increase in overpressure can lead to a very narrow drilling window.

All these issues need to be assessed pre-drill and captured in the well planning process, considering most likely and high case pressure predictions and optimal positioning of casing to maintain wellbore stability, drilling safety and minimize potential rig downtime.


How high pressure happens

High pressure in rocks can occur for many different reasons - including because the rocks still contained water when they were buried (and the water then expands); gas generation; clay mineral reactions in shales; collapse of the rock's load supporting framework.

The biggest factor of these is rocks containing water when they were buried - in other words, the sediment was buried quickly.

For a high rate of sediment burial, river systems depositing large volumes of suspended sediment at their deltas are the most likely locations, the most visible of which are the Niger and Congo Rivers exiting into the South Atlantic.

Tertiary deltas such as these typically exhibit a high sand content on the shelf and delta top, leading to near normal pressure conditions down to depths of 3000m+ (10,000 feet), below which sharp increase in overpressure leads to several drilling challenges.

The start of overpressure at relatively shallow depths of burial (typically 1000m (3000feet) below sea bed) leads to a long and continuous narrow drilling window, requiring frequent casing strings to maintain borehole integrity.

Some wells along the continental margin of West Africa have had to set total depth prior to reaching all their expected targets due to these extensive pressure transition zones.

Further seawards, and especially down the continental slope, the sediments become more mud-rich and any sand reservoirs are confined to channel and fan systems, most likely enclosed in low permeability shales. The pressure profile in more continuous shale sections is one of constant increase in overpressure, often with a gradient running parallel to the overburden. From a drilling perspective the rate of increase along the pressure transition zone is gradual with less probability of a drilling surprise.


Predicting high pressure

Traditionally the pressure of shale-rich rocks is predicted by analysing seismic and existing well data.

You work with the principles that

(a) high pressure is associated with higher than expected porosity

(b) the parameter used to capture porosity is of good quality with high data density

(c) the only reason for the overpressure is compaction disequilibrium (a high rate of sediment burial).

In one example from offshore West Africa, 4 log data types (sonic, density, neutron and resistivity) have been used from a single well to estimate the pressures in a thick shale section. There was good consistency of patterns of pressure estimation in the shales using all four log types (see image).

Since shale pressure cannot be independently verified, except in very shallow sediments (under 200m burial) it is imperative to build confidence of the overpressure magnitude, since drilling any sediments with high permeability will lead to a wellbore influx (potential 'kick') if the mud weight is not high enough to balance the formation fluid pressures.

High confidence comes with consistency of results, combined with understanding the reservoirs and their relation to inter-bedded shales (and other lithologies).

Isolated reservoirs often share similar overpressures with surrounding shales, whereas inter-connected reservoirs can allow pressure transfer leading to both higher and lower pressures than the surrounding shales (poor calibration for shale-based models).

Pressure prediction becomes more challenging and uncertain where pressure generating mechanisms other than compaction disequilibrium are contributing to the overpressure. The confidence of any prediction in which multiple mechanisms are active is lower than for disequilibrium compaction alone.

From a pre-drill prediction point of view, the inability to anticipate contributions to overpressure from fluid expansion and chemical processes and the associated modification of the rock properties used for pressure profiling can lead to substantial under-prediction of reservoir pressures and hence add substantial risk to the safety of well operations.


Regional studies

Where sufficient density of wells is available, compilation of the data basin-wide offers immense advantages.

Ikon GeoPressure has demonstrated in Europe, USA and most recently in West Africa, how understanding the bigger picture from their regional studies helps to extract both improved prediction capability from local data (thereby reducing risk) and enhanced exploration opportunities.

Successful pressure prediction does not rely only on local rock property data (such as porosity from velocity, density or resistivity) coupled with direct pressure data from offset wells, but also must correspond to a realistic 'model' for the development and distribution of basin fluids in the subsurface over geological time.

Commercial fully coupled basin flow modelling software offers a way to gain insights into fluid flow behaviour during progressive sedimentation, burial and the influences of temperature.

Basin models are particularly well suited where seismic definition is poor (for example under salt).

Building a model for an area of 100km2 is likely to take 3-4 weeks with the same amount of time to test and migrate from 1D through 2D to 3D.

The outcome will be a set of maps and plots which describe the distribution of overpressure in all the main reservoirs and their relationship with each other.

Each of the wells has been examined not only for direct pressure data, but also for their relationship with shale pressures from a standard pressure prediction interpretation (after testing for which mechanisms are active geographically and varying with depth and temperature).

For Ikon Science's work studying pressure in the Niger Delta, the entire data package provides a unique description of the sub-surface plumbing of the Niger Delta. In addition the data is rich enough for basin-wide compaction, overburden and fracture gradient algorithms to be generated and then tested.

The Deep-Water Niger Delta project was completed in Q2 2011, after which other areas of the Niger Delta will be the prime focus.

In addition to reduced risk (better definition of the drilling window based on both pore pressure and fracture pressure prediction) the description of the subsurface pressure relationship benefits exploration in areas such as seal breach risk (top seal failure prediction) as well as potential trap definition associated with fault sealing.

Other areas of the West African margin which could benefit from regional or semi-regional pressure studies of this nature, include offshore Mauritania, Ghana, Angola, Gabon and Cameroon.

Mapping of overpressure and recognition of pressure compartments related to faults help to establish which faults preferentially leak and which seal, adding confidence to the search for downthrown fault traps.

In the same way top seal failure can be assessed by examining the relationship between the pore pressure expected in the reservoir and seal, and the fracture strength of the top seal.

Another direct exploration benefit relating to understanding regional pressures occurs where reservoirs are connected over a large area and communicate to the surface, thereby drawing down the deeper reservoir overpressures relative to their intra-formational shales.

These conditions are known in the North Sea and South Caspian basins and similar geological conditions are found along the West African margin.

The lateral drainage from these reservoirs sets up a hydrodynamic flow through the reservoir over geological time. The changes of overpressure resulting from lateral drainage can be mapped.

Ikon GeoPressure is currently engaged with the Nigerian federal authorities, DPR, Sonar Ltd and Napims, and with a group of six sponsor companies, in mapping pressures across the Niger Delta. Phase 1, covering the deepwater and ultra-deepwater areas is due to be completed in April 2011. Phase 2 (continental shelf and nearshore areas) and Phase 3 (onshore and swamp) will be completed between 2011 and 2014.