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Chapter 13 – DRILLING PROBLEMS AND SOLUTIONS                     317






                 second possibility is that the hydrostatic pressure exerted by the mud is
                 insufficient to hold the formation together, and slivers of shale fall off into

                 the well as cavings (mentioned in chapter 3). This also can occur when
                 drilling in overpressured shales. The pore pressure might even be higher
                 than the mud hydrostatic pressure, but the well does not kick due to the
                 very low permeability of shale, and high levels of cavings result. Even if
                 the annulus is clean of cuttings at the start of a trip out of the hole, reactive
                 formations can cause material to fall in during the trip and stick the pipe.
                 The warning signs are that the force needed to pull the drillstring out does
                 not decrease as rapidly as would be expected as steel is removed from the
                 hole. If this occurs, the driller must stop pulling out and circulate the hole
                 clean again.
                    In shallow, unconsolidated sands, tophole collapse might occur. The
                 hydrostatic pressure must be able to support the formation, but if there is

                 not a good filter cake and losses occur, there is little difference in pressure
                 between the formation around the wellbore and mud hydrostatic pressure.
                 The sand simply falls into the well. (This problem can be demonstrated by
                 trying to dig a deep hole on a beach.) The mud must have good plastering

                 characteristics  so  that  a  firm  filter  cake  is  formed,  allowing  the  mud

                 hydrostatic to hold the sand back.
                    Sometimes, formations are fractured. This can happen especially with
                 brittle shales, coal, and limestone. Mud gets into the fractures, and this
                 lubricates the fracture faces as well as changing the pressure regime near
                 the wellbore. Pieces of formation fall off (fracture cavings). Cavings from
                 fractured formations can be identified by their size, shape, and presence of

                 fracture faces that may be visible.
                    Junk in the hole can also fall into this category. If something is lost
                 down the hole, either because of something breaking or by something
                 falling through the rotary table, stuck pipe may result.
                    Cement that is not fully set is known as green cement. If a cement plug
                 was set in the hole and the driller runs into it with the bit while it is still
                 green, the pipe can become plugged or can get stuck in the cement.


                    Differentially stuck pipe

                    Differentially stuck pipe is far less common today than 15 or 20
                 years ago (fig. 13–5). The knowledge of the mechanisms involved, better






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