Well-Bore Stability 4/4

PROVIDING A STABLE WELLBORE

Presence of the following should be mitigated by remedial action;

·        Indications of tectonic activity in the area.

·        Sudden pressure transition zones expected.

·        Adverse formations expected (reactive shale, unconsolidated or fractured formations, abnormal or subnormal pressured zones, plastic formations).

·        Wellbore inclination greater than 30 degrees.

Determine Magnitude of In Situ Condition

·        Obtained from density logs of offset wells.

·        Estimated by seismic and logs.

·        Determined by LOT and/or logs.

·        Rock strength is estimated through correlations with sonic density logs since slow sonic velocity and high porosity generally relate to lower rock strength.

Research Offset Wells for Indications of Stability Problems

Offset well data is invaluable information for identification of stability problems in the field.

·        Identify hole sections with stability symptoms.

·        List the conditions that caused the stability problem.

·        Identify similar problems in offset wells occurring at the same vertical depth. Look for similarity in the conditions that caused the problem.

·        List the drilling parameters effecting the problem (i.e., mud type and weight, hole angle, adverse formations, unusual drilling practices).

Avoiding Stability Problems

·        Select an inhibitive mud for reactive formations.

·        Casing points should allow for mud weight windows determined from stability analysis.

·        Maintain mud weight/ECD in stability window.

·        Use down hole ECD monitoring tools in critical wells.

·        Optimize well trajectory based on drilling days vs. stability.

·        Plan for effective hole cleaning and stuck pipe prevention.

·        Follow defensive drilling practices.

Warning Signs and Corrective Actions.

No single action can prevent stability problems and so wellbore stability must be managed by the controllable parameters:

·        Mud type, composition and density.

·        Drilling practices (minimize ECD, swab /surge pressures).

·        Wellbore angle and direction.

·        Chemical stability problems occur when reactive shales are drilled with a non-inhibitive drilling fluid. Chemical stability is time dependent and difficult to quantify. The drilling fluid interaction results in shale hydration and swelling which leads to shale falling into the wellbore causing hole enlargement and tight hole conditions.

Chemical Stability Problem Indicators.

·        BHA balling and slow drilling, flow line plugging, soft mushy cuttings on shaker.

·        Smooth increases in torque/drag

·        Overpull off slips, pump pressure increasing.

·        Increases in mud parameters (mud weight, plastic viscosity, yield point, cation exchange capacity (CEC), and low gravity solids).

Preventive /Corrective Measures

Chemical stability problems are prevented by selecting proper mud type and composition. Initial corrective measures are to use suitable mud additives. If the problem persists, replacing the existing mud with a more inhibitive mud may be necessary.

·        Addition of various salts (K, Na, and Ca) to balance water activity.

·        Addition of glycol to reduce chemical attraction of water to shale.

·        Addition of various "coating" polymers (PHPA, etc.) to reduce water contact with shale.

·        Use of oil base or synthetic oil base mud to exclude water contact and entry into shale.

·        Minimize the open hole exposure time.

·        Plan regular wiper trips.

·        Minimize surge/swab pressures.

·        Ensure adequate hydraulics for bit and hole cleaning.

·        Maintain required mud properties.

·        Use minimum mud weight, if possible.

Mechanical Stability

Mechanical instability is related to incorrect mud weight /ECD and/or well trajectory. Too low mud weight can cause hole cavings or collapse resulting in stuck pipe. Too high mud weight /ECD can cause excessive fluid losses to the formation or total loss of returns. Typical warning signs are;

·        Large size and volume of cavings over shakers.

·        Erratic increase in torque/drag.

·        Hole fill on connections or trips.

·        Stuck pipe by hole pack-off /bridging.

·        Restricted circulation /increases in pump pressure.

·        Loss of circulation.

Loss/gain due to ballooning shales.

Two indicators of mechanical stability problems are loss of circulation and increased volume of cavings. Partial or total loss of circulation may be due to pressure induced or naturally existing fractures. The reduced hydrostatic associated with loss of circulation may cause formation caving and collapse.

Preventing Mechanical Stability Problems

The constraints on wellbore pressure are dictated by formation pressure on the low end and fracture strength on the high end. Hydraulics planning must also consider minimizing the shock load imposed to the wellbore. Measures to prevent/correct mechanical stability problems include:

·        Increase the mud weight (if possible). The mud weight values should be determined using a stability analysis model and past experience if drilling in a known field.

·        If drilling fractured formations, it is not recommended to increase MW. Increase the low end rheology (< 3 RPM Fann reading). Improve hole cleaning measures. Maintain 3-rpm Fann reading greater than 10. GPM for high-angle wells equal to 60 times the hole diameter in inches and half this value for hole angle of less than 35 .

·        Circulate on each connection.

·        Use back reaming and wiper trips only if hole conditions dictate.

·        Minimize surge/swab pressures.

·        Monitor torque/drag and the size and amount of cuttings on shakers.