Occasionally the drilling fluid pressure exceeds the fracturing pressure and drilling fluid is lost to the formation. Several types of lost circulation materials (LCM) are used to heal such losses. However, in standard testing procedures like American Petroleum Institute Recommended Practice 13-1 or 13-2, only a 100psi or 500psi differential pressure is required in the pressure cell for LCM testing. It is shown that this pressure is by far too small to give any meaningful data for LCM performance.
Lost circulation can occur in all well sections. Especially, will lost circulation represent a potential problem during drilling long sections with varying formation strengths and varying formation pressures. In a typical drilling situation, the minimum static overbalance in the well is more than 100psi in order to control the formation pressures and avoid influx of formation fluids into the wellbore. During drilling this overbalance will increase due to, amongst others, frictional effects. These effects, together with variations in formation pressures sometimes lead to dynamic differential pressures exceeding 2000 or even 3000psi. Testing under conditions exceeding the expected maximum differential pressures are meaningful in order to identify the sealing capabilities of LCM materials under such conditions.
Some tests focus on losses through tapered slots. In such cases particles larger than the slot outlet will sooner or later contribute to stopping the loss. In a real situation the fracture would be in position to open further. Hence, such tests are not optimal. Other tests are based on slot openings as the minimum size. In such cases the fluids will need to bridge off the opening to work. Based on bridging formation ideas, a development of new LCMs was conducted. These LCMs were able to handle downhole pressure differences.
Slotted disks were installed into a high-pressure cell. The slots were 18.00 mm long and 400, 700, 1600, 2000 and 2500μm wide which were made into disks with a diameter of 24.13 mm. Drilling fluid was pumped through the cell and an LCM filter cake was formed across the disk slot. The pressure required to break this filter cake was obtained (unless it exceeds 5000 psi) and recorded. The fluid filtration losses through the apparatus was strongly dependent on the LCM concentration. A set of LCM tests was performed, and examples are given where the LCM actually would withstand a differential pressure 5000 psi across the slotted disks without failing.