Access Midstream Partners LP (NYSE:ACMP) is progressing from last week’s rating of B (“buy”) as the company improves to an A (“strong buy”) this week. Shares of ACMP have increased 18% over the past month, better than the 0.2% the S&P 500 has seen over the same period of time. For more information, get Portfolio Grader’s complete analysis of ACMP stock.

Ecopetrol (NYSE:EC) shows solid improvement this week. The company’s rating rises from a B to a A. Ecopetrol engages in the exploration, development, and production of crude oil and natural gas. Wall Street has pushed the stock higher by 9.4% over the past month. The stock has a dividend yield of 4.4%. For more information, get Portfolio Grader’s complete analysis of EC stock.

Chesapeake Midstream Partners (NYSE:CHKM) ups its rating to a B (“buy”) this week after earning a C (“hold”) in the week before. Chesapeake Midstream Partners owns, operates, develops, and acquires natural gas, natural gas liquids, and oil gathering systems, as well as other midstream energy assets in the United States. For more information, get Portfolio Grader’s complete analysis of CHKM stock.

Nexen Inc. (NYSE:NXY) earns a B this week, jumping up from last week’s grade of C. Nexen is an independent global energy company that is based in Canada. For more information, get Portfolio Grader’s complete analysis of NXY stock.

Rentech Inc. (AMEX:RTK) is seeing ratings go up from a B last week to a A this week. Rentech develops and licenses a proprietary and patented process for the conversion of carbon-bearing gases, liquids, and solids into liquid hydrocarbon products. For more information, get Portfolio Grader’s complete analysis of RTK stock.

Louis Navellier’s proprietary Portfolio Grader stock ranking system assesses roughly 5,000 companies every week based on a number of fundamental and quantitative measures. Stocks are given a letter grade based on their results — with A being “strong buy,” and F being “strong sell.” Explore the tool here.