Optimisation of Solids Control Opens Up Opportunities for Drilling of Depleted Reservoirs

Proper knowledge on what type and size distribution of particles to be used to strengthen formation is necessary to
successfully apply this technology. However, it is equally important to control the particle content on the offshore
application. Shaker performance and the mechanisms for wear on shaker screens must be known to control that only
desired particles are returned to the well16. The particle size distribution must be quantified. A practical measurement tool for measuring the particle size distribution of drilling fluids has been developed. In the following a field test where drilled solids and added particles are allowed to be recirculated and thereby naturally create formation strengthening is described. The test is based on the solids control methods and equipment described by Dahl et al. and is using the particle size distribution equipment presented by Omland et al.

Drilling through depleted reservoirs is a challenge since the pressure difference between the fracturing pressure and the pore pressure becomes small; or even sometimes negative. During the last decade a technique has been suggested that increase the fracture strength towards the original strength, and, in some cases the new fracture strength has exceeded the original strength. This method is based on fracturing the borehole wall with small fractures and then fill these with impermeable particles to stop further propagation of the fractures at the same time as the fractures remain increasing the formation strength of the remaining portion of the borehole wall. The concept of increasing the formation strength while curing lost circulation has been discussed by Messenger and Morita et al. Fuh et al. suggested using this method while drilling to prevent lost circulation. A suggestion of the selection of material and the theoretical treatment were later refined by others.

Theoretical analysis and all analysis of field experience on formation strengthening conclude that it is necessary to
optimize the particle size distribution of the added solids. The fracture must be sealed by a non-permeable easily plugging material. The plugging is caused by arching or gel formation or a combination of these two items. Arching in pipes and conical sections has been a large subject for research the last century, and the results are included in most textbooks on soil -or powder mechanics. Although this subject is well established theoretically, there is still a need for experimentally optimizing the particle size distribution and particle content. Furthermore, for drilling fluid applications this particle size distribution must be optimized with respect to drilling fluid viscosity profile, gel formation and gel fragility, viscoelastic properties as well as chemical properties of the fluid and particles. In the laboratory there is therefore a need to develop proper equipment to evaluate different selections of added particles. An example of such a device has been presented by Hettema et al, and this device has been applied to further improve the particle size distribution.

Field case

The active use of solids control equipment as a tool to control the particle size distribution of solid particles in drilling fluid was first utilised during drilling the reservoir section of a well on an HTHP field offshore Norway. The pressure depletion in the field due to production was estimated to be approximately 100 bar; causing a reduction in the fracture gradient. The previous well drilled in this reservoir suffered from massive losses of drilling fluid because of exceeding the fracturing pressure. Effective down hole drilling fluid density could not be reduced sufficiently to compensate for the reduced fracture gradient due to inhomogeneous reservoir with possibilities of drilling into zones of virgin pressures. A strategy to be able to drill this formation was therefore to add particles to the drilling fluid system in order to enhance the fracture strength during drilling these depleted zones.

The 8 ½” section was drilled using a caesium formate brine based drilling fluid system. The particles added to the
system were limited to bridging particles for fluid loss control against the permeable formation and particles added for fracture strength enhancement. This particle blend consisted of a blend of different grades of CaCO3, graphite and nut shells. The total concentration of particles added was limited to 140 kg/m3 and distribution was selected based on laboratory optimisation against both slot openings of varying size as well as porous ceramic disks. This optimised blend was added as the initial particle content in the fluid. As drilling commenced the drilling fluid also picked up drill solids that will be incorporated as a part of the particle blend of the fluid and in addition particles were removed over the shale shakers. Prior to drilling a plan was made in order to monitor and control the particle content and size distribution of the particles. A premix consisting of high concentration of coarse particles
was made. This premix was bled into the active system in order to compensate for the coarsest particles removed on the shale shakers. The rate of addition was determined based on using various laboratory techniques to ensure that the fluid system contained sufficient particles to seal off any induced fractures.

The planned method of using the solids control equipment actively to control the PSD was thought to be made possible by controlling the feed to each shaker. All shakers were dressed with different sets of screens creating the desired PSD. Fig. 1 shows the proposed initial shaker screen configuration for the different shakers to obtain this effect. Due to difficulties in handling the flow, the concept was left and the fluid run over coarse screens to keep the particles in the fluid system. For drilling of short intervals and/or with simple fluid systems this strategy might prove suitable. This can though create challenges if drilling longer sections and where fluid adherence to the cuttings needs to be avoided. In such cases the concept of using the shaker screen configuration actively for controlling the PSD might show to be necessary in order to control fluid properties and limit the necessary treatment of the fluid.

In order to use the shale shakers efficiently to control the PSD the shakers must be engineered correctly and the screen selection must be done correctly. Analysis of the Cutt point of different types of shaker screens has identified big discrepancies between the manufacturers cut point and the actual cut point. Fig. 2 shows the measured error for different shaker screens at different given Cutt points. This will influence the final PSD of the fluid going through the shakers, thus the shaker screens to be used for the purpose of making a given PSD of particles in the fluid passing through must be thoroughly selected. In addition the wear of the screens in use must be followed closely so that broken screens are displaced with new ones as soon as possible.

In order to control the feed rate of new particles and the fluid flow over the different shaker screens continuous PSDanalysis were performed offshore during drilling using a laboratory sieve stack. A reference test was run prior to
drilling out of the 9 5/8” shoe, as shown in Fig. 3.

As can be observed, the PSD changes rapidly during the drilling phase even over relatively short length intervals. This illustrates how the drilling operations affect the particles in the fluid systems continuously throughout the drilling operation. The measurements given indicate a continuous decrease in cumulative amount of particles in the system as drilling progresses even with particle additions. It also demonstrates a relatively decrease in the concentration of coarse particles due to the drill string grinding effect. In sum this indicates that the feed rate of new particles was too low.

Simultaneously the fluid was tested using a modified production screen tester (PST) for plugging ability. The
amount and time for the fluid to pass through artificial fractures of 500 and 1000 microns are given in Table 1. Even
though the amount of coarse particles decreased the fluid was still efficient in plugging even the biggest slot size of 1000 micron. The slot sizes used in these tests were chosen based on rock mechanics estimations of expected fracture sizes for the given reservoir depletion.

PPT (Permeable Plugging Test) were also run for measuring the plugging ability towards formation. Tests were
run using 20 microns discs at 500 psi differential pressure and the reservoir temperature of 150 oC. The results of these tests are given in Table 2. As shown from the results the fluid loss control was good throughout the drilling. The coarse particles were not expected to have big influence on the fluid loss results.

PSD-analysis were also performed on samples sent to onshore laboratory. In these analysis a fully automated laser diffraction apparatus was utilised. The results are given in Fig. 4.

These analysis give a more continuous analysis throughout the different sizes, also measuring the small sized particles that were added for fluid loss control against the pore openings of the sandstone formation. The analysis show a shift in the PSD from coarse to finer from start of drilling. The PSD measured after 58 and 93 m are relatively similar, but with less coarse material than at the start point. This indicates that the addition of coarse material was too low in order to keep up with the loss of these particle sizes. A similar development was seen using the sieve stack offshore as shown in Fig. 3 above. At 93 m a treatment was performed bringing the PSD back on track. At TD of the section the fluid was screened to take out the coarsest particles prior to running the liner. This is clearly demonstrated in the PSD analysis of this sample.

Use of particle additions for formation strength enhancement has long been recognized to efficiently minimize lost
circulation incidents. Despite significant research in the area, one is still reluctant to trust the solids control equipment and/or continuous particle additions to have the same effect in the field. For application where the solids control equipment is used for removal of drilled solids, the efficiency of this is likely to influence the effect of particles for formation strength enhancement. That is, having poor control of the screen wear might in many cases benefit the operation as more particles are re-circulated back to the bore hole. For complete process control this is not an appropriate approach.

Application of different measurement techniques for determining the particles’ ability to plug formation during this
field trial provides complementary information to the current available techniques. This can be used to determine how different operations affect the fluid’s capability for plugging the formation, whether it is optimization for lost circulation prevention, reduction of formation damage or for other purposes. Until recently, these techniques have not been applied actively in the field and thereby question the meaning of extensive laboratory studies optimizing ideal particle blends which will not be the field composition only after a short period of drilling. Due to discrepancy in results from the various tests performed, further investigation should be performed to reveal the weaknesses of each test method in combination with further development of techniques than can ultimately monitor the particle type and content continuously.

The field trial further illuminates the importance of active use of the solids control equipment for continuous control of the particles recirculated. Currently the solids control equipment determines the actual Cutt-point and is the only
approach for controlling the type and amount of particles in the fluid system.

From the study performed, the following can be concluded:

• Particle additions aid minimising lost circulation incidents.
• The effect of particle additions can not be directly derived from the case study, but through another approach this can be achieved.
• The drilling operation significantly affects the particle size distribution.
• The amount of particles in the fluid system requires alternative monitoring techniques that provide the particles’ plugging ability.
• The shaker screen configuration can be used actively to ensure correct size distribution and amount of
  particles in the system if used properly.