Electrical conductivity is a parameter that can be used to monitor the entire hardening process of oilwell cement slurries. The theoretical relationship among conductivity, porosity, cement chemistry, and ion content is discussed. The theory is confirmed by experiments; the decline in the slurry conductivity is primarily a function of porosity decrease and, thus, the degree of hydration. The applied results show that the electrical conductivity of curing slurries reflects strength development and that rapid hydration will reduce the risk of gas migration.

Settling and sagging of barite in inclined boreholes may lead to safety and operational problems. To study the effect of rheology on settling, a laboratory tool was designed, consisting of two connected tubes, one inclined and one vertical. The hydrostatic pressure was measured at the bottom of each pipe. Stable and unstable muds can clearly be differentiated through their pressure behaviour. Several muds were studied at simulated static and dynamic conditions. The results show that sagging is most severe during laminar flow and also indicate that the rheological parameters may be used for predicting stability problems. Introduction In weighted drilling mud barite tends to segregate slowly. In directional drilling operations the settling process is accelerated. Barite settles in the lower side of the borehole and starts sliding when the borehole has an inclination above 30 °. This phenomenon is known as barite sagging. Sagging can lead to drilling and completion problems; a density variation or non-linear hydrostatic pressure gradients which can lead to pressure control problems, while thick and tight barite beds can lead to high torque and drag, stuck pipes and plugged boreholes, and even lost circulation. The sag problem is related to the so called Boycott effect(1), first described in 1920. Hanson et al.(2) have investigated the phenomenon and found that most of the sagging occurs while the mud is circulating. The same conclusion was reached by Bern et al.,(3) the sagging tendency is highest at low annular velocities. Zamora and Jefferson(4) presented a method for tracking drilling fluid density variations, which helps to detect, but not to predict drilling mud instability. Jamison and Clements(5) developed a test method to characterise settling and sag tendencies in static drilling fluids, however their equipment was not able to distinguish between settling and sliding. They also found that their data, based upon standard API rheological parameters like PV and YP, were unsuitable for prediction of sagging behaviour. It is apparent from previous works that the most difficult part of the problem is the prediction of drilling mud instabilities for both static and flowing drilling fluids. To date there are no API test procedures for sag testing. A new simple laboratory tool was therefore designed for the purpose to study and develop a method to predict sagging. Drilling Mud Separation Settling of Particles In vertical wells the settling of weighting material is generally not a problem due to the long settling distance. In horizontal wells the distance to the lower side of the wall is only about 0.2 m, which leads to rapid generation of solids beds. The settling velocity of a single spherical particle, vs,l, in a fluid is expressed by Stoke's law(6): Equation 1 (available in full paper) A barite particle (ρp = 4,200 kg/m3) with a diameter (dp) of 20 µm in a fluid of density (ρfluid) 1,500 kg/m3 and a viscosity (µ??of 40 cP will settle at a rate of 53 mm/h.

Particles like barite and cuttings influence the friction properties of a mud. The standard API lubricity tester, however, cannot measure friction of fluids containing particles, and to overcome this problem it was modified with a cam setup. It was found that particles indeed alter the friction. Large beads are being used to reduce friction. They are, however, filtered out in the solids control equipment and to avoid this, we have investigated smaller polymer microbeads which will pass unhindered. The microbeads reduce the friction in water based muds with around 40% which is significantly better than four commercial lubricants.

Refraction seismics with the shotpoints and the hydrophone cable on the seabottom, have become the standard geophysical method for investigating rock quality before constructing offshore tunnels in Norway. In connection with the construction of a sub-sea tunnel by the Norwegian Public Roads Administration, research work was carried out to compare two low-velocity zones, indicated by refraction seismics with other methods. A special resistivity cable for pole-dipole measurements on the sea-floor, with 10 m between the electrodes, was constructed. A 200 m long profile, crossing the two low-velocity zones, was measured with all combinations of electrode distances. The two zones were detected as low-resistivity zones. A special data processing technique to enhance the anomalies is described. Resistivity soundings in a seawater environment to detect fracture zones in the bedrock underneath the bottom sediments, are discussed. It is concluded that severely fractured zones, which may cause difficulties for the tunnel construction, can be detected both with sea-surface and sea-floor arrays using long electrode spacings.

FIGURE 1: Principle of the shrinkage measurements.FIGURE 2: Total shrinkage of slurry T25. FIGURE 4: Total shrinkage of slurry W90. FIGURE 3: Total shrinkage of slurry T60. FIGURE 5: Total shrinkage of slurry W140.