Commonly used drill-in fluids (DIFs) typically contain starch, xanthan, and sized calcium carbonate. Although DIF is inherently less damaging than conventional drilling mud, relatively impermeable filter cakes are deposited on the borehole wall. A common practice to minimize such damage is applying acid (i.e., external breaker) or a strong oxidative breaker (i.e., internal breaker) system to dissolve filter-cake solids and biopolymers. Magnesium peroxide is an internal breaker and is classified as an oxidizer, which decomposes slowly to release oxygen. The magnesium peroxide, when exposed to an acidic solution, releases hydrogen peroxide that degrades the polysaccharide-type polymers and removes the external filter cake.
To realize the full potential of openhole horizontal completions, formation damage caused by residual filter cake must be removed. A common approach to mitigate such damage is application of strong organic acids, ester-generated organic acids, chelates, enzymes, and combinations of these or different oxidative breaker systems to dissolve/destroy filter-cake solids and biopolymers. Two basic methods are applied to deteriorate filter cake that contains polymers. • External breaker placed in contact with the surface of the filter cake. • Internal breaker deposited as an integral component of the filter cake.
Magnesium peroxide is a very stable strong oxidizer and can be used as an internal breaker. At medium or low temperatures, it remains inactive when added to polymer and sized-solids DIF, and thus becomes an integral part of the filter cake as it is deposited. According to the particle-size distribution and active-material content, there are many types of magnesium peroxide available. It is best if the selected type matches with the pore-size distribution of the drilled reservoir section and with the drilling rig’s solids-control system. The particle-size distribution is affected by the volume and particle-size composition of the other bridging materials also. Because of the extremely low solubility of magnesium peroxide, it remains stable for extended periods of time while in an alkaline environment and within the filter cake. Upon contact with hydrochloric acid (HCl), the solid peroxide decomposes to form hydrogen peroxide. Hydrogen peroxide generates in-situ oxygen, which attacks starch and xanthan polymers. Autoxidation occurs as the polymer is exposed to the oxygen. Experimental Approach To evaluate the effect, laboratory experiments were carried out before performing field trials. The effect on various muds was evaluated. A range of mud systems was selected for evaluation. Six DIFs were formulated on the basis of the field application. Procedure for Filter-Cake Deposition.1. Weight of 5-darcy ceramic disk was measured
The rheological data showed that addition of magnesium peroxide had no effect on the DIF rheological parameters. Hence, it can be used safely in the mud formulation. The best cleanup result was obtained in the case of Formulation-5, as seen in Fig. 3. The magnesium peroxide influenced acid cleanup. DIF that includes the new breaker system has shown excellent stability and cleanup after 15% HCl soak. On the basis of these promising laboratory results, field trials in three wells were carried out to ascertain the effect under field conditions. Field Trial The target reservoir for the trials was a South Omani sandstone. The mature and relatively heavy 19°API oil is highly depleted in this part of the field. Formation permeability is between 200 and 2,000 md, formation temperature is approximately 50°C, and the formation pressure is 4000 to 6000 kPa at 900 m depth. There were mud losses at many places while drilling, with dynamic losses on the order of 4 to 6 m3/h and static losses on the order of 1.5 m3/h. Well 1. The pay section was drilled to 2080 m without drilling problems. The well was completed with a liner. Before displacement of mud with brine, magnesium peroxide mud sample was collected and a mudcake was obtained by use of a ceramic disk and filtering for 6 hours at 100-psi pressure. No HP/HT filter press was available, so the API filter press was used. The filter cake was soaked in 15% HCl. The cake was removed completely. After the confirmation test, the openhole section was soaked with 15% HCl. Observed fluid loss was 1.5 m3/h. The predicted oil production from this well was 44 m3/d, but actual production was below expectations. Well 2. During drilling of the pay section, partial losses of approximately 3 to 6 m3/h were observed, which increased to 18 m3/h. Being a barefoot completion, 31/2-in. tubing was run into the well and a single-stage acid soak was performed. During soaking, the observed acid loss was 32 m3/h. During drilling of this part of the hole, a filter-cake sample was obtained. The filter cake was removed completely after soaking. Also, the predicted oil production of this well was approximately 30 m3/d, but actual production was 43 m3/d. Well 3. Except for 10 to 15% caving, no other problem was encountered. The caving could be controlled by increasing mud weight to 11.3 kPa/m3 and use of a modified-sulfonatedasphalt treatment. The section was drilled with the same mud formulation used in the previous two wells. The horizontal section was drilled to 2082 m, with partial losses of 3 to 5 m3/h observed. The losses could be cured with a lost-circulation-material pill containing medium and fine calcium carbonate. After lowering the liner, the cleanup was performed by soaking twice for 2 hours with 15% HCl. During soaking, acid losses of 12 m3/h were observed. As for previous wells, a mud sample was collected during drilling and filter cake was obtained by filtering in an API filter press for 6 hours under 100-psi pressure. Then, the filter cake was soaked in 15% HCl for 2 hours. The filter cake was removed completely after soaking. The predicted oil production of the well was approximately 50 m3/d, and actual production was 58 m3/d. Conclusion An effective filter-cake-removal treatment was required for filter cake formed by drilling-fluid components including polymer and calcium carbonate. The magnesium peroxide breaker system removed the filter cake successfully. The damage factor after removal of the filter cake with magnesium peroxide breaker was demonstrated by the production results.