- High losses of drilling fluid through the solids control units.
- High volume of dilution.
- High volumes of fluid processed requiring strictly regulated disposal to protect the environment.
- Continuous plugging of the mesh in the solids control units.
- Increase in the consumption of screens.
- Fluid overflow due to plugging of the screw conveyors.
- Poor supervision of the solids control units.
- Increased operating costs.
Solids control equipment performance during drilling operations is affected by many factors. The conventional array of solids control units consists of a cascade-type screen configuration, hydrocyclones, mud cleaner, decanter centrifuges and a drying shaker (Fig. 1). While drilling near surface sections, the predominant presence of gumbo-type clay causes continuous plugging of the mesh in the screens, which, combined with the high rates of penetration typical while drilling these intervals, affects efficiency in handling the circulating volumes.
In late 2001, PDVSA-Oriente expanded its continuous improvement processes for drilling operations to include solids control optimization as recommended by the drilling fluids service company on location. The optimization process was applied to wells drilled north of Monagas (Fig. 2) and began with the incorporation of a flowline distributor at the end of the return line and a new configuration of the primary screens of the solids control system. The changes introduced made it possible to reduce the problems associated with handling fluids while drilling highly reactive clays.
Selection, capacity and position of the units depend on characteristics such as flow volume, type and properties of the drilling fluid, penetration rate, and the lithology of the formations encountered. Based on operational experience, solids control service companies attempt to continuously improve the equipment and incorporate high performance units in order to maximize mechanical removal efficiency during drilling.
During the second phase of the evaluation, considering safety aspects in the operations and the negligible benefit obtained by retaining the screw conveyors and dryer shaker, it was decided to eliminate these components.
Case Study 1: First Optimization Phase
Phase 1 of the optimization process was initiated on wells 1 and 2. A flow distributor at the end of the return line and a linear configuration of the array of the screens (Fig. 3) were introduced, while maintaining the number of units typical of conventional arrays. As both wells were drilled, monitoring SCE performance was limited to measuring efficiency by mass balance every three days (Table 1). The improvements introduced during this first phase led to the following results:
- Uniform distribution of the fluid on the screens.
- Optimization in the design and consumption of mesh during the operation.
- Reduction of fluid volume lost and requiring disposal.
- Improved supervision of SCE performance.
- Reduction of operating costs.
Table 1 : SOLIDS REMOVAL EFFICIENCY: PHASE 1 EVALUATION AND OPTIMIZATION PROCESS
| Interval Drilled
|Mechanical Removal Efficiency
|Well 1||26||790||92.56||100% Sand|
|17½||3.486||90||80% Sandstone, 10% Shale and 10% Siltstone|
|8½||19.580||88.87|| 60% Sandstone, 20% Siltstone, 10% Shale, 10%
|Well 2||26||720||91.89||100% Sand|
|8½||14.645||88.26||60% Shale, 40% Sandstone|
A comparative study of the data obtained was used to establish a technical justification for using a dryer shaker. From the analysis, it was determined that the use of the dryer shaker did not present any advantage for the operation since, in the majority of the cases, the MOC values for the discarded cuttings from these units were similar to the MOC values
achieved by the primary and secondary screens and those obtained with the primary units (Fig. 4).
Case Study 2: Second Optimization Phase
The second phase of the optimization process was carried out in wells 3 through 9. Wells 3, 4, 6 and 7 belong to the same field; wells 5 and 8 are located in nearby fields, and well 9 is exploratory, in a different field from those previously mentioned.
The application of the new focus began in the second phase, consisting of the application of an evaluation matrix with the following parameters: fluid density and rheological properties; total solids content (high and low gravity); percentage MOC, in volume and weight; classification of the mesh design and mechanical removal efficiency. The evaluation matrix was applied daily and daily execution of retorts was required.
Solids control unit
Well 3 was the first of this drilled in this optimization phase, after the screw conveyors and dryer shaker had been eliminated from the system (Fig. 4). High performance screens were incorporated. In all the wells, continuous records were maintained reflecting those variables included on the evaluation matrix. Close monitoring made it possible to establish and improve the ranges of operation of the SCE and the MOC in weight of the drill cuttings processed, as well as to validate the decision regarding the use of high performance screens with finer mesh than had been traditionally used in each one of the phases, and eliminating the dryer shaker and screw conveyors. This phase also included the direct discharge
system, consisting of a series of inclined metal trays designed to move the solids discharged from the screens, mud cleaner and decanter centrifuges to the collector tanks.
While drilling of the near surface interval with water base fluids, the high performance shown by the SCE (average 5.7 “G” power in the mud screens) during the evaluation made it possible to use finer mesh in the primary mud screens: 175x175x175 mesh arrangement in the primary units and 250x250x250 mesh in unit 3 in 1. In the past, it had been difficult to drill these phases with high flow volumes (>900 gpm) using 84x84x50 mesh designs in the primary units and 175x175x175 mesh in unit 3 in 1.
While drilling the intermediate hole with oil base fluid and where the predominant lithology of the formation consists of shales, the operation implemented an optimized mesh design (210×210 mesh for the primary units and 250 mesh in the 3 in 1). In the production interval, where the predominant lithology consists of sandstone and shale lenses, the same design of mesh described for the intermediate hole was used, thus optimizing the mesh design for the primary mud screens which traditionally functioned with the 175x175x175 mesh array and 250x250x250 for the 3-in-1 mud cleaner.
Due to continuous monitoring, it has also been possible to improve the quantity of mud screens recommended for drilling. In wells 3 through 6 and 9, five primary mud screens were used, and, in wells 7 and 8, four primary mud screens were recommended and used, obtaining good performance in the solids control units.
The improvements described above made the following possible:
- Elimination of operational risks, with associated losses of time for electrical and mechanical failures.
- Elimination of risks, in matter of safety, associated with the operation of the screw conveyors.
- Reduction of costs associated with the solids control process.
- Improvement of the mesh design.
- Reduction of the volume of fluid associated with solids discarded. Better control of the MOC percentage.
- Establishment of acceptable MOC levels, measured in percentage by weight.