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Groundbreaking to the Core

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Coring Risks

 

The data obtained from core is used widely and ultimately affects fundamental decisions about how to proceed with a well and field. Once you add in rig time and the cost of a core analysis programme, coring can become an expensive business and have a relatively high cost per foot. With this in mind as many precautions as possible need to be taken to minimise risks to the core and maximise the quality of the data obtained from it. Risks and impacts to core data can most likely occur during the following stages:

  • The Coring Process

  • Tripping

  • Lay down, handling and wellsite analysis.

 

Risks during Coring - Jamming

 

Jamming is one of the biggest risks to coring and can be caused by a number offactors that include formation, BHA, and Bit Design :

  • Formation fault slant – formation causing the classic wedging jam action

  • Fractured formations

  • Unconsolidated core material collapsing under its own weight and causing high friction against the inner barrel.

  • Expanding clays causing high friction and adhesion against the inner barrel.

  • Outer barrel stabilisation

  • Compression loading

  • Bending of the outer barrel leading to bit whirl, bounce and other negative effects

  • Core bit configuration, design and dynamic factors

  • Bit Whirl – leading to undergauge core, fractures and disking.

  • Weight On Bit – too high WOB can induce fractures ahead of the core bit.

These factors can be mitigated by the experience and knowledge of the core hand in the field as well as a properly designed coring programme, and utilisation of correct downhole equipment and corehead.

 

Risks during coring - Other

 

Other factors that can affect the quality of data during include:

  • Flushing of the core by drilling fluid resulting in filtrate invasion. Filtrate invasion can displace the native fluid, change the in situ fluids and alter rock properties.

  • Bit whirl resulting in undergauge core.

  • Poor parameter choice leading to low quality core

These risks can also be mitigated through proper planning, experience, correct parameter choice, equipment selection and mud design.

 

Tripping Out/Pull Out of Hole 

 

Pull Out Of Hole is another crucial stage for coring. The pull out of hole can have a negative impact on core quality if not done correctly. A pull out of hole schedule should be devised as part of the coring programme and must be followed. The tripping times given in the programmen are slip to slip and the pull should be constant not a quick pull and then wait.

If the POOH rates are not followed correctly then the gas expansion can cause some serious damage to the core:

  • Fluid and gas expansion – as the pressure reduces as the core comes to surface gas and fluid expansion can cause serious damage to the core and its properties. Rapid gas expansion can cause structural damage to the core, seriously affecting the data obtained from the core, potential invalidating it.

  • Gas expansion also results in oil shrinkage, oil and water expulsion, extruded core and deeper fluid invasion.

  • This damage can be mitigated by following the correct pull out rates, as well use the use of pressure vented inner barrels.

 

Surface Handling and Processing

 

In a lot of ways the greatest risks to core quality occur once the core is on surface.

  • Separation of inner tubes – Torque induced damage.can be mitigated by use of a non-rotation breakout system.

  • Laydown of core barrels - Flexing can induces fractures. This is mitigated by use of Core Laydown Cradles.

  • Transfer of core– This can be mitigated by using a spreader beam to fully support the core during lifting operations.

  • Cutting the core - Core can be damaged during the cutting process this is mitigated by the use of a band saw.

  • Core Handling – Special attention should be taken by personnel handling core. Poor core handling can seriously damage core.

  • Core Shipping – Shipping of the core is a risk business. Impacts during lifting operations can induce fractures, as can shocks during road freight. In order to minimise risks during shipping the core should be stabilised at the rigsite. It should then be packed into specially designed shipping containers that protect the core during transit. A shock monitor can also be used to monitor the core during transit to record the time and degree of impact of shocks. This data can be used to identify areas where the chain of custody needs to be tightened up.

 

Care needs to be taken at every stage of the coring process. If the core is damaged then it can dramatically affect the quality of the data obtained during the analysis programme. This in turn can have dramatic effects on the viability of a well and eventually this can have serious financial implications, as can be seen on the following page.