NEW METHODS FOR MUD SOLIDS CONTROL OPTIMIZATION ON THE RIG

More sophisticated mud systems in the North Sea has drastically changed the concept of mud solids control on the rig. Getting a “better handle” on mud costs and reducing the pollution associated with losses on surface have become major concerns for the Operator and have resulted in a focus on the efficiency of mud mechanical treatment equipment.

This topic shows how an extensive and full scale test campaign has allowed a Drilling Contractor to develop new methods for the optimization of the use of shale shakers I mud cleaners I centrifuges. After a brief review of laboratory results, it describes field trials and shows how the use of sophisticated measurement devices and on site computer evaluation can lead to significant improvements in typical North Sea system operations.

INTRODUCTION

The major equipment manufacturers convinced Sedco Forex that there was a need to improve the theoretical understanding of Mud Solids Treatment and to raise the level of expertise needed to operate a rig system. In 1985, Sedco Forex decided to initiate an extensive research program on Mud Solids Control Process. This program was conducted in three different steps.

Full Scale laboratory testing of Solids Control Equipment using the Schlumberger Cambridge Research Centre facilities. Definition of representative “performance curves” for each piece of equipment.
Field Testing of the Equipment in the North Sea. Verification of test results as per Step 1. Definition of optimized sample taking methods and measurements.
Definition of New Methods at rig level for the improvement of mud handling procedure and the optimization of Solids Control Process. – Development of refined computer software.

The Research Program was completed early in 1987, results/ conclusions are now directly applied on the field by implementing a new type of service for Mud Solids Control optimization and oil spillage monitoring.

FULL SCALE LABORATORY TESTING OF SOLIDS CONTROL EQUIPMENT

Shale Shakers / Hydrocyclone / Mud Cleaners and Centrifuge performances were evaluated in preselected oil and water base mud. These tests were designed to fulfill the following main objectives:

Situate each type of solids control equipment in terms of:

×Flow rate capacity

×Cut point

in various typical water and oil base mud characteristics (Tables 1 and 2).

Compare and operate preselected makes of each kind of solids control equipment.
Evaluate applications of field procedures to check and quantify solids control equipment efficiency.

Table 1. SUMMARY OF MUDS/RHEOLOGY USED FOR CUI POINT TESTS

	Bentonitic water base muds (12m₃)			Invert emulsion low toxic oil base muds (12m₃)
Mud weight (PPG)	8.7	12	14.2	9.2	12.3	14
Plastic viscosity (CP)	12	17	22	27	35	51
Yeild point (PA)	5	11	15	10	15	18
Hematite (%V/V)	0	8	14	3.5	12	17
Total solids (% V /V)	5	14	20.5	14.5	21	24
Oil/water (%)	–	–	–	80/20	85/15	85/15

Table 2.SUMMARY OF MUDS/RHEOLOGY USED FOR FLOW RATE IESIS

	Bentonitic water base muds (12m₃)				Invert emulsion low toxic oil base muds (12m₃)
Mud weight (PPG)	8.6	11.5	12.8	15.1	8.6	10.6	12.6	14
Plastic viscosity (CP)	13	21	30	50	41	50	50	68
Yeild point (PA)	10	11	16	30	13	14	15	27
Hematite (%V/V)	0	6	12	20	0	6	12	18
Total solids (% V /V)	5	11	17	25	5	11	17	24
Oil/water (%)	–	–	–	–	66/34	66/34	77/23	84/16

Shale Shaker Tests

Two high performance shakers were tested (one double deck circular motion / single deck straight line motion shaker) in order to:

Compare cut points of respective shaker screens.
Check cut point variation with mud properties.
Evaluate I compare flow rate capacities and monitor the wetness of cuttings and the amount of high gravity solids discarded.
Check resistance of screens to plugging.

Hydrocyclone / Mud Cleaner Tests

Seven different 4 inch diameter cones were tested. Cut points of cones for various mud rheology and solids content were determined.

Two mud cleaners, one equipped with rectangular pretensioned mesh screens and one equipped with circular pretensioned mesh screens were evaluated. Tests were designed to compare their efficiency in terms of low gravity solids removal while checking the wetness of the cuttings and the amount of high gravity solids discarded in the same mud conditions as the shale shakers.

Centrifuge Tests

Two centrifuges were tested (one with high particle acceleration I one with low acceleration). These tests were designed for evaluation of the influence of the different operating parameters on the efficiency of the centrifuges. Applications such as “solids removal“, “rheology control”, “cutting cleaning” using centrifuges were appraised with different types of mud.

Summary of Result of Laboratory Tests

We selected three results that are of primary importance for the choice of the best solids control line of equipment according to mud characteristics.

Result 1

Cut points do not vary significantly when mud rheology changes. Figure 1 shows that the separation curve of a 120 mesh screen in a 9ppg, (1.08s.g.), 10 centi-poise water base mud is very close to the curve in a 9ppg, 36 cp oil base mud.

Flow-rate is significantly modified when mud rheology changes. In contrast, the flow rate showed that for the same 120 mesh screen, the flow rate capacity of shaker A in a 9ppg, 36 cp oil base mud, is almost a third of what it is in a 9ppg, 10cp water base mud.

Result 2

Figure 2 shows that the denomination used for layered screen can be confusing. The API screen mesh designation cannot be applied to complex weaves and to date, there is no standard procedure to describe layered screens as opposed to single layer screens. The cut point comparison is vital to make reliable conclusions on comparative shale shaker flow rate capacities.

Result 3

Figure 3 shows the variation of the separation curve of a 4 inch desilter cone with mud characteristics (density, viscosity). Although variations in the separation curve between the different makes of hydrocyclones were significant, especially in heavy and viscous mud, the curves given here were close to the average results. It is easy to realize that the results are very far from 15 microns D50 cut generally advertised for a 4 inch desilter cone.

Based on those results it is possible to make the following observations.

As far as cut point is concerned the desilters are only efficient in un-weighted non viscous water base mud.
The overall separation of a mud cleaner is poorer than the separation of its cones {part of the solids from the cones underflow is recycled by the screen) . The comparison of the curves of figure 1 and 2 shows that except for the 9ppg water base mud, a shale shaker with a 120 mesh screen will make a better separation than the 4 inch cones. Eventually, even the 100 mesh screen will make a better cut. This shows that an additional shaker on the flowline, which will allow the use of finer screens, may be an efficient substitute for the mud cleaner in most applications.
The use of screens, finer or equal to 150 mesh on a mud cleaner in weighted water base mud, may be a waste considering the poor separation of the cyclone. Such a screen will discard more weighting agent for a minimum removal of drill cuttings. In many cases, a 120 mesh screen under the cones will remove almost as much· solids and discard much less barite for a better screen life. One could wonder why, with such a poor cut point, is there any particle of weighting agent discarded in the cone underflow. However, according to White (ref. 5) in a 9ppg mud, a particle of barite of diameter d will settle and therefore be discarded in the underflow at the same speed as a drill cutting of diameter 1. 45 In addition to this the hydrocyclone separation curve flattens when the viscosity and the solid content of the mud increase. Therefore, even when D50 cut point of the cone is poor, a significant amount of barite might go out through the cone underflow.

FIELD TESTING OF SOLIDS CONTROL EQUIPMENT

After the full scale laboratory tests were completed, the next step was to launch field tests on offshore rigs in order to upgrade the laboratory results and define with a greater precision the limits of operations of the different pieces of equipment in actual operating conditions.

Sophisticated measurement equipment and additional qualified people were moved to the North Sea where a nine months test campaign was conducted in both the North and South of the UK section (oil-base mud drilling). This campaign has been completed in February, 1987.

Measurement Equipment

A mud laboratory was installed on the rig for each of the tests. Sophisticated measurement equipment allowed analysis of samples with refined accuracy.

Precision Scales (0.01gr accuracy)
Laser diffraction particle size analyzer (For centrifuge testing).
Sieve Shaker
Rheometers
Evaporation plates and retorts.

The major problem encountered was the measuring of sample weights to precision. Although results were adequate, in most cases, a satisfactory solution at 100% was never found when very rough sea conditions were encountered.

Flow meters were also added (on one rig) to monitor the fluid additions to mud pits in order to monitor better oil/mud transfers and evaluate accurately where losses occurred.

Sample Talking / Measurement

Samples were taken at·. each stage of the mud solids treatment process

Shale Shaker = Feed – over-screen – under-screen.

Mud Cleaner = Feed – under-cones – over-screens – under-screens.

Centrifuge = Feed – discharge

Particular care was given to solids discharge where the samples were collected over long enough periods of time to give the most accurate value possible of the discharge mass flow-rates (this is necessary to evaluate the equipment efficiency with a good precision and to cope with eventual variations).

Evaporation methods were selected for solids % determination (rather than retorts) as this allowed a better accuracy in sample weight measurements. For each sample, the following information was gathered –

Sample specific gravity
Mass Flow-rate
Rock density
Oil content
Solid % (low gravity solids and barite)
Particle size repartition (sieving and/or laser particle size analysis.

This allowed to calculate accurately equipment and to define criteria to parameters or switch them on/off the efficiency of the adjust their operating.

Data Process

A huge amount of data was collected during the field test campaign and it became necessary to use computer facilities for analysis/plots and calculations. A relational data base was created in the Sedco Forex Research and Engineering department on a Vax 780 computer from which the statistics of: equipment operating parameters, mud characteristics and sample analysis results were obtained. (Figure 4). Interpretation software including graphic routines to plot separation and flow rate curves were developed. It allowed a permanent follow up and checking of the quality of the results.

Summary of Results of Field Tests

The North Sea field tests allowed several key achievements:

Determination of the limits of operations for the equipment in actual field conditions =

1.1. Shale shakers are the key components of an adequate mud solids system and the new “high performance” shakers allow significant reduction on mud cost when expensive mud system are used. However, their efficiency varies a great deal depending upon several factors:

The feed mass flow-rate (mud flow rate and amount of cutting drilled).
The type of formation drilled (major problems being encountered through unconsolidated sand lithology due to the plugging effect.)
The particle size and shape (depending upon the lithology drilled and the type of bit used).

The separation curve for a given screen mesh is not significantly affected by a change in mud characteristics such as mud type, mud weight or plastic viscosity·. To the contrary, flow rate capacity decreases as plastic viscosity or mud weight increases.

The tests also enabled definition of guidelines for monitoring the cutting oil wetness (screens selection on cutting oil wetness criteria).

1.2. Mud cleaner performance limitations are quite significant in particular when using weighted systems:

Hydrocyclones make a very poor separation on a weighted mud and recycle undesirable solids through the over-cones – (Results found in laboratory tests were confirmed by the field results).

The barite particles tend to conglomerate and the carry over (or piggy back) phenomenon on drilled solids is significant. A large quantity of the barite particles falling onto the mud cleaner shaker are not passed through the screen as intended, but, instead, are discarded with the drilled solids. (Example figure 5).

The solids are sheared through centrifugal pumps and hydrocyclones. The test results showed that this “particle damage” was not negligable and that the % of fine particles in the active was building up significantly when mud cleaners / hydrocyclones were used continuously.

1.3. Centrifuge performances were analyzed in terms of flow-rate, cut-point and wetness of solids discarded for different applications –

Solids removal (unweighted muds)
Rheology control (double stage centrifuges)
Cutting cleaning

A common idea with centrifuges, is to compare their performances in terms of mud feed capacity, a concept which may be misleading. For example, one centrifuge may have a greater fluid capacity than another but removes less dry solids if the cut point is higher or if the solids are wetter. To avoid these problems of inconsistencies, centrifuge efficiency was defined in terms of flow-rate of dry solids discarded. Guidelines for designing equipment layout and selecting operating parameters were drawn up. Some of the observations from the centrifuges field tests are as follows.

Solids Removal ( Unweighted Muds )

Centrifuges are efficient for solids removal in unweighted muds – operating parameters need to be adjusted by taking into account the following test results.

The mass flow-rate of dry solids discarded is proportional to the feed flow rate – therefore, the maximum solids removal capacity corresponds to the maximum feed flow rate capacity.
The influence of pool depth, bowl speed and differential speed on the solids discharge capacity is very small compared to the influence of the feed flow rate.
The feed flow rate is the major parameter affecting the cut point curve.

Rheology Control ( Double Stage Centrifuge)

The purpose of this process is to reduce the viscosity of a weighted mud without noticeable reduction of the mud weight and without discarding the expensive liquid phase. This process is based on the fact that solids in the range of 2 to 5 micron are the ones most detrimental to viscosity and must be removed.

One field test result showed that the “double stage centrifuge system” had very little affect on the mud rheology. First the range of detrimental solids has never been confirmed and, the cut point of the two centrifuges were very similar with only a small amount of slurry removed by the second centrifuge.

(Note: Same results were obtained during full scale laboratory tests.)

Cutting Cleaning

Data shows that good results can be obtained, providing that the equipment is monitored efficiently (as shown in figure 6 – when not properly monitored – a centrifuge may discharge wetter cuttings than the mud cleaner over-screen discharge). The equipment lay out is always the key factor for satisfactory results. An intermediate slurry tank needs to be built so that cuttings are kept in suspension for centrifuge processing It must be carefully sized and the fluid dilution flow-rate must be adjusted depending upon the mass flow-rate of cuttings discharged to be processed.

Conclusion from Field Tests

Field tests showed that there was a need to approach the solids treatment on the rig from a more scientific point of view than is usually done. A better understanding of the process, a better knowledge of each equipment limitation and a close monitoring of the system on hand, gave a continuous and accurate evaluation of each piece of equipment, efficiency allowing a significant minimization of mud losses (up to 45% on one of the tests). This lead to the defining of new methods for monitoring the mud treatment process on board the rig.

METHODOLOGY FOR RIG SITE IMPROVEMENT OF MUD TREATMENT PROCESS

Optimization of the mud treatment process is required to reach these main objectives –

To get lower solids % in the mud (therefore better mud characteristics and fewer down-hole problems).
To minimize the volume of mud lost on surface and decrease the total mud bill.
When drilling with oil base mud, to decrease pollution and record more accurately the oil discharged in the sea.

Significant results can only be obtained if decisions are taken from a scientific point of view and if extra means in terms of extra personnel and equipment are put on the rig.

There is a need to define a “mud handling program” which is part of. the standard drilling program with three phrases – namely, the planning phase, the execution of the programme and the end of well report.

The Planning Phase

The contractor reviews the operator’s mud program i.e., type of mud, specific gravity profile and rheology characteristics. The purpose of this study is to check that the rig equipment layout is appropriate for the forecasted operations. (For example, are centrifuges required?) Previous laboratory and field test results facilitate the recommendation to the operator of corrective steps, if needed. An initial operating parameter program is set (it includes type of screens to be used – determination of zones where equipment efficiency will need to be calculated for decision making, etc.) Problem areas – such as unconsolidated sands or salt formations – are identified.

This program is reviewed and discussed during the rig pre-spud meeting.

The Execution of the Program

It is not sufficient to assign extra people on the rig (such as Mud Solids Control Technicians), to make the program successful – there is a need to create a new team spirit within the whole rig crew. A new organization is set up on the rig in order to achieve this goal. (Figure 7). The contractor Rig Engineer – under the responsibility of the Rig Superintendent – leads the “Mud Solids Team” and makes sure that a good communication exists between all the parties involved in mud monitoring (i.e., contractor crew, solids control team, mud engineer). Each of . the rig personnel feels more involved in the optimization of mud handling as clearer objectives and responsibilities are defined.

The rig preventive maintenance program ensures that each piece of the solids control equipment is in good order. Daily check lists ensure that immediate action is taken in case of faulty devices or malfunction.
Standard procedures are applied for any mud transfer (especially when oil base mud is in use) to avoid mishandli.ng and/or discharge overboard. Loading/back loading of boats is always performed under the responsibility of the Barge Engineer (or designated representative). (By applying such methods, significant reductions in the mud volumes lost due to surface leaks and/or bad handling procedure have been observed.)
A mud laboratory is installed on the rig for the optimization of the solids control process. This labor a tory is equipped with all the required measurement equipment, (similar to the ones used for laboratory and field tests) and with a computer for data processing and analysis.

Samples of mud and discarded slurries, are analyzed on a regular basis so that the efficiency of each piece of equipment can be calculated. A crosscheck of results with the previous SEDCO FOREX tests ·data, allows the definition of the most appropriate operating parameters. (Figure 8 shows a simple example of monitoring mud cleaner operating parameters). By applying the “mud handling program” (prepared during the planning phase), a new situation is created on the rig. The rig crew is able to plan the way they will monitor the equipment and will be ready for any change in the drilling conditions, (change in lithology, ROP, new bit type, run in bole, etc.). As a consequence, significant mud savings can be achieved. (For example, shale shaker screens will be replaced by coarser mesh a few feet before drilling an unconsolidated sand formation where a plugging problem is to be encountered. Less mud will be discharged overboard.

Standard daily reports are issued by using the laboratory computer, (UKOOA forms in the North Sea), so that the general performance can be continuously evaluated. (These reports are

to be compiled in the end of well report).

The End of Well Report

The End of Well Report is compiled at the contractor’s office. It includes the analysis of all the data recorded during the well operations and allows evaluation of the efficiency of the Mud Solids Control at each stage of the operations as well as where the mud has been lost on surface. Recommendations for optimization during future operations are also prepared.

In the case of development drilling, this report will be the key document used when preparing the “Mud Handling Program” for the next wells to be drilled (Planning Phase).

Conclusion

There is today, severe pressure on operators to reduce, in real terms, the costs of their wells. Therefore, optimization is becoming even more than in the past, the new challenge for the operator and the contractor. Optimization of mud handling operations and mechanical treatment is a key part of this challenge.

Sedco Forex North Sea experience shows that significant gains in reduction on the total mud costs can be obtained by applying more efficient methods for the monitoring of mud solids control. A 25% to 30% reduction of the volumes lost on surface can be achieved (up to £70,000 for a 12,000 feet well/oil base mud drilling). When oil base mud is used, as a direct consequence, pollution is decreased as less oil is discharged over board. Additional costs – induced by extra people and equipment required on the rig – have been proven both economically and environmentally worthwhile.