DRILLING

Well Bore Cleaning ProceduresINTRODUCTION

Hole cleaning is one of the main issues when drilling wells, whether in cased or open hole. Well sections with high inclinations have been identified in the lab and confirmed in the field to be the zones with cuttings sedimentation and recirculation problems. In these wells, hole cleaning is often perceived as a straightforward process, but quantifying cuttings retrieval rates and determining the residual bed height downhole should be a management process involving two challenges: decaying and re-circulating cuttings, and transporting solid particles to the surface.

These challenges are further linked with the compatibility between operating parameters, wellbore quality, wellbore stability, safety margins and cost-efficiency of the operation.

By optimizing hydraulics, mechanical parameters and drillstring construction, it is possible to increase performance of: drilling, operating cost, safety margins and drilling risk prevention from stuck pipe, lost circulation and hole instability.

HOLE CLEANING PRINCIPLES

How do we determine when a hole is clean? The usual approach is to monitor the shakers for changes in the cuttings retrieval rate. Although this is a direct indicator, it does not provide complete information and can lead to incorrect assumptions and detrimental misunderstandings. The quantity of retrieved solid particles may not represent the actual volumes that should be retrieved due to irregularities in the hole section. Even if the cuttings volume remaining in the hole looks acceptable, the distribution of cuttings along the hole can have a significant impact on drilling efficiency. An irregular distribution profile can induce avalanching, packoff, pressure surges and wellbore instability.

Hence the importance of defining successful hole cleaning by using a more complex monitoring system and taking into account risk-occurrence analysis. Monitoring systems should capture the cuttings retrieval rate, Equivalent Circulating Density (ECD), pressure losses and torque and drag curves. Risk occurrence analysis should include pressure surges, back reaming, hole packoff, casing/equipment wear and stuck pipe.

HOLE CLEANING AND DRILLING PRACTICES

Hole cleaning problems start when operating parameters fail to efficiently circulate cuttings to surface. Problems have been identified in the drilling and tripping phases.

Drilling and tripping represent two different modes of cuttings bed buildup processes and hole-cleaning practices. During drilling, there is an equilibrium cuttings-bed height that can be used for hole-cleaning efficiency by measuring the cuttings at surface versus the calculated cuttings volume expected to be generated when drilling a formation. Even though the bed reaches a steady state (provided parameters remain unchanged), the cuttings-bed height is not necessarily regularly distributed along the drillstring. For this mode, hole-cleaning performance corresponds directly to the final equilibrium bed height.

During tripping, cuttings can build up under several conditions. Natural sedimentation of solid particles occurs when mud flow stops. Dragging the drillstring through the existing cuttings bed creates localized dunes. Cuttings avalanching occurs when the bed is established in the critical angle (45-60) section of the well. For this mode, hole-cleaning performance is related to the speed at which the system decays the cuttings and to the final cuttings-bed height.

WELL BORE CLEANUP METHODS

Displacement of the oil mud from the wellbore uses a combination of mechanical equipment (scrapers, circulating subs etc.) with chemical spacers. Once obm is out of the wellbore, returning fluid is assessed visually to decide if it is clean enough to be discharged, or retained within the limitations of pit space.

Run scrapers and junk baskets for the cleanout run to remove debris and reduce circulating time.

Use circulating subs for higher pump rates to give a cleaner interface.

Use pipe rotation (possible with some types of circulating sub) and reciprocation to reduce fluid dead-spots providing a cleaner interface.

Jet BOP cavities, a potential source of debris, which may prolong the need to circulate. Can be cleaned running in the hole with or without specialist tools.

Boost the riser once displacing fluid is above the BOPs where low annular velocities can lead to an extended interface especially if heavy mud is displaced by seawater.

Review options for Choke and Kill lines on Semi-Submersibles containing obm, which may need to be displaced to brine before obm is out of the riser. With a negative liner lap test, Choke and Kill lines may be the preferred means of displacement despite low flow rates and no rotation. The riser must then be displaced with the potential for dead-spots between rams.

Maximize flow rates for cleanest interface. As the interface returns, flow rate is slowed to observe cleanliness of displacing fluid prior to diversion or discharge.

Review single or two stage displacement options to minimise fluid contamination. Single stage - obm to completion brine directly, two stage - obm to seawater then completion brine. Pit logistics, or negative liner lap test may need obm to remain on board to displace the well in the event of a leak.

Consider packers as an alternative to a negative liner lap test, which can impact the logistics of many operations.

Measure directly oil content of the seawater/brine. Currently tests to define oil content take at least 20 - 30 minutes. Although returns can be diverted to a mud pit before evaluation, space may be limited. NTU readings do not correlate with oil content. Samples can be retained for evaluating future strategies.

Spacers.

Minimize spacers - typically base oil, surfactants (detergents), solvents, high viscosity, low viscosity, and flocculent spacers. Volumes, sequence and effectiveness vary. Whilst this is an important area for minimizing volumes there are a wide variety of strongly held opinions.

Treat as produced water. Large spacer volumes and types may limit options in later treatment e.g. potentially oil contaminated seawater could be treated as produced water, but detergents preclude this option because of their effect on treating equipment. Avoid detergents if using oil absorbent filters. Avoid viscous spacers if using oil absorbent filters.

OPERATING PARAMETERS ANALYSIS

Flowrate is the main component of hole cleaning. In most cases, required flowrates will not be achievable due to the available rig capability. Further flowrate limitations are restricted by maximum ECD values on the formation. In addition, high flowrates can generate hole enlargement or caving. Increasing the flowrate to improve hole cleaning can be done within limitations, but care must be taken to avoid worsening the situation.

Rotational speed (rpm) is the second most well-known, and sometimes misunderstood, hole-cleaning component. Although increasing pipe rotation improves cuttings agitation, drillstring rotational speed has a limited effect on cuttings recirculation and cannot achieve a completely clean hole.

The primary advantage of increased rpm is cuttings agitation, which limits cuttings sedimentation. The lifting effect is created by the upward driving effect of mud-flowing lines on the solid particles. Therefore, rpm efficiency is related more to cuttings agitation than cuttings recirculation.

Increasing rotational speed increases the risk of drillstring fatigue and dynamic vibration. It also increases the risk of wellbore damage and caving. In addition, it has been shown that high rpm will increase ECD values, mainly due to drillstring behavior at high rotation speed in conjunction with mud-flowing line disturbances. Increasing the rotational speed to improve hole cleaning is applicable for low-complex wells and is of limited efficiency. However, field experience has proven that string rotation above 120 rpm avoids excessive cuttings sedimentation, thanks to particle agitation by the drillstring.

Mud rheology plays a significant role on cuttings suspension in the flow. The lubricity of the mud is the main factor that helps suspend cuttings. Mud rheology is not an optimization parameter, since it will not be variable. Mud properties are adapted to the formation and well profile.

Hole cleaning performance was monitored based on:

Cuttings: Monitor cuttings regularly gauging their volume, size and shape to indicate hole cleaning effectiveness.

Torque and drag: Ensure that pick up, slackoff, rotating-off-bottom weight and torque data are recorded in a consistent manner after each stand. This is interpreted real-time on the rig floor for immediate action. If a tight spot is encountered (>30 kips overpull) while tripping out of hole, assume the tight spot is from cuttings. Run-in-hole two stands, until the BHA is clear of the obstruction. Circulate and rotate for 30 min. or longer to confirm that the tight spot is caused by a cuttings bed.

Weighted pills: Regularly pump 14-ppg pills as the hole dictates to sweep cuttings beds. Record sweep effectiveness in the drilling reports. If a large amount of cuttings are seen on the shaker, then increase pill frequency and/or reduce ROP, so as not to overload the annulus.

CONCLUSIONS Most of the industry has identified hole cleaning problems while drilling deviated and extended-reach wells. The main operational benefits from this new hole-cleaning approach were measured directly while drilling: minimized back reaming operations, no stuck pipe or hole enlarging risks.

Using the new, mechanical hole-cleaning system, achieved enhanced hole cleaning at selected levels of flow rate and rpm values, allowing optimized levels of ROP and pressure losses. Field performance analysis showed torque and drag reduction due to enhanced frictional properties of the HBZs. Better hole cleaning reduced the remaining cuttings-bed height and decreased cuttings accumulation risks.

Prior to drilling highly deviated or complex wells, the hole cleaning system must be accurately pre-determined to ensure that all aspects of cuttings transport are reviewed. The full involvement of the drilling team, when implementing the cleaning system.